I think you are attacking it from the wrong angle by trying to encode all posted data.
Note that a "<" could also come from other outside sources, like a database field, a configuration, a file, a feed and so on.
Furthermore, "<" is not inherently dangerous. It's only dangerous in a specific context: when writing strings that haven't been encoded to HTML output (because of XSS).
In other contexts different sub-strings are dangerous, for example, if you write an user-provided URL into a link, the sub-string "javascript:" may be dangerous. The single quote character on the other hand is dangerous when interpolating strings in SQL queries, but perfectly safe if it is a part of a name submitted from a form or read from a database field.
The bottom line is: you can't filter random input for dangerous characters, because any character may be dangerous under the right circumstances. You should encode at the point where some specific characters may become dangerous because they cross into a different sub-language where they have special meaning. When you write a string to HTML, you should encode characters that have special meaning in HTML, using Server.HtmlEncode. If you pass a string to a dynamic SQL statement, you should encode different characters (or better, let the framework do it for you by using prepared statements or the like)..
When you are sure you HTML-encode everywhere you pass strings to HTML, then set ValidateRequest="false" in the <%@ Page ... %> directive in your .aspx file(s).
In .NET 4 you may need to do a little more. Sometimes it's necessary to also add <httpRuntime requestValidationMode="2.0" /> to web.config (reference).
If you set turn page's ViewState off, then there is no way for you to enable ViewState for specific components. This is because ViewState is serialzed recursively, so when if the Page is not allowing ViewState, it will not serialize the ViewState for any of it's child controls.
In answer to your question, if you don't want to explicitly turn ViewState off on individual controls, but want to keep some controls ViewState aware, the best way would be writing a small utility method which turns ViewState off for all controls (using recursion or otherwise). Then enable ViewState for the controls that you would like to enable ViewState for.
Alternatively, a middle ground and less forceful way may possible if controls are groups inside other container controls (such as Panel). You can disable ViewState for all controls inside a Panel by disabling ViewState of the Panel.
Best Answer
The __VIEWSTATE field is also used to store control state, which is not optional. Furthermore, the information contained in the view state is used to validate the postback, if I'm not mistaken (and validation is enabled, which is the default). So as long as you have the form with runat="server", you'll have a viewstate field. However, you should notice a much smaller field size if you disable all viewstate.