Edited 2017 - changed recommended long life storage voltage and added comments on fast charging using some recent systems. RM.

What YOU do as regards several of these questions depends largely on what YOU are trying to achieve or test.

Discharge to cutoff is fully discharged (to whatever remaining % that voltage represents). That's the easy one :-)

Percent dropoff of current in tail sets final % of max possible charged reached. There was a superb table given here within last week or so. Can supply later if you don't find it.

Real Men™ plateau at 4.2V and tail down to 10% or even 5% of the constant current rate. This gets the battery full and knocks the stuffing out of it.

Others terminate the current tail at say 25% of cc value.

Optimum lifetime for ongoing usage is at about the end of the constant current phase. That makes it very easy to locate - charge at specified current until desired max voltage is reached, then charge at constant voltage as desired. Here "desired" is to stop immediately. This is the point at which batteries tend to give significantly longer whole of life mAh of storage without grossly reducing mAh capacity per cycle. This is liable to be the point where older "fast chargers" tell you they have finished. Actual % total claimed varies but probably 70% - 80% range.
Newer USB input fast chargers use the term differently. In the case of USB the maximum available charge current at 5V is 5A so that the battery MAY be able to be charged at ~= 6A for the CC part of the cycle using an efficient buck converter to drop voltage and raise current.
[For a buck converter: Vout x Iout = Vin x Iin x efficiency_of_conversion]
Some systems such as QuaqlComms Quick Charge system allow the use of higher charger voltages (9, 12, 20) with specifically designed equipment, so battery charging can be faster for a given voltage provided that the battery specification allows this.
Maximum charge rates for LiIon and LiPo batteries are usually C/1 = 1A per Ah of battery capacity.
At 5V, 5A a USB charger can charge a 6000 mAh 1 cell LiPO battery at max rate - so eg a 10,000 mAh single cell battery used in some larger tablets can not be charged at the allowed 10A ! rate.

For long life storage where actual stored capacity is unimportant, LiIon and LiPo cells should be stored at about 3.7V.

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Using cells without protection adds to the rich tapestry of life. As long as you don't mind the occasional scorch mark on the tapestry that's fine. Note that part of the protection is a one time high capacity fuse under the cap for when things get out of control. Undervoltage discharge destroys. Charging from below a certain voltage at full rate can get fun, I'm told. Charging at reduced rate can bring cell up, I'm told. Below another second level they say don't even think about it. I've had very poor success in trying to get LiIon to misbehave. I have a box of unprotected cells that are very uncooperative about venting with lame etc. Strange. Sony and Apple and even HP seem to be much better at it :-).

90.5% @0–100% SoC

for electrolyte type A

89.1% @0–100% SoC

for electrolyte type B

I read reverse calculated this from graphs in a non-public measurement report where 2 different types of battery electrolyte (solid/gel) technologies were compared for a 0 to 100% SoC (state-of-charge).

Battery measurements were taken by a technical university, at 21ºC room temperature during 1 month, the year 2013, comparing the results from 150-180 discharge/charge cycles. Batteries under test were all in the range between 5 and 10Ah.

• charging curve: CC-CV
• charge to discharge point: V = 14.375V and I < 0.30A
• discharge to charge point: 0% SoC
• resting period: not specified
• 0% SoC was defined as discharge voltage reached 10.8V (1.8V per cell)
• 100 SoC was defined as charge voltage reached 14.375V (2.396V per cell)
• charge was done at a rate of 0.3C (+/- 0.05C)
• discharge was done at a rate of approximately 0.1C (fixed resistor was used)

The result between charging and discharge power was (remarkably) about 1.6Wh for both battery types, where one battery had a remaining capacity of approximately 40Wh and the other 20Wh.

Some data read back from the graph (resulting in lower accuracy) for electrolyte technology A:

Cycle   Charge [Wh]   Discharge [Wh]
1       100           91
2       100           88
3       100           86
4        97           81
5        92           76
6        86           73
7        81           69
8        78           67
9        74           64
10       73           63
11       70           61
12       69           59
13       65           56
14       63           55
15       61           52

136      44           40
137      44           40
138      44           40
139      45           40
140      45           40
141      43           40
142      44           40
143      45           40
144      44           40
145      44           40

According to my opinion the last measurements are more significant than the first measurements due to the initial formation process. Average last 10 cycles power charged into the battery is 44.2Wh. Average last 10 cycles power discharged from the battery is 40.0Wh.

Some data read back from the graph (human graph point via line to value translation) for electrolyte technology B:

Cycle   Charge [Wh]   Discharge [Wh]
1        85           75
2        83           72
3        80           70
4        78           69
5        76           67
6        76           67
7        75           67
8        75           67
9        74           66
10       74           66
11       73           65
12       73           66
13       72           66
14       72           65
15       73           64

136      25           22
137      25           22
138      25           22
139      24           22
140      25           22
141      24           22
142      25           22
143      25           22
144      25           22
145      24           22

Average power charged into the battery during last 10 cycles is 24.7Wh. Average power discharged from the battery is 22.0Wh.