Both are used. Note, words in brackets (...) are optional.
 You can watch TV whenever you want to (watch TV).
 You can watch TV whenever you want (to watch TV).
Choice stems from a prescriptivist rule that tells us never to end a sentence with a preposition. Mind you, "to" of "want to" isn't a preposition. It's a infintive marker, but it's homophonous with (sounds the same as) the preposition "to".
 Regardless of whether you want to (attend) or not (want to attend), you should (attend).
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