You only have to untaint data you attempt to use in a critical operation. If you're using tainted data in, say, your open calls, unlink, system, etc., you need to un-taint your data first. If you're just printing data or using it for decision-making, you don't have to untaint it.

This doesn't apply to just form data. Any data that is retrieved from outside of your script is potentially bad and potentially made harmful. Perl marks it as tainted, and will prevent you from trying to use it in any critical operations, since the effects could be adverse.