I don't use dechlor on the betta tanks, I've had no ill effects whatsoever. But before any of you say anything, yes, if something bad did happen (and it's pretty easy to tell), there's plenty of dechlor around.
Well if you know your tap only contains chlorine, letting it sit and evaporate for a few hours usually works too. If your tap contains chloramines then it's a different story. Chlloramines affect fish differently than chlorine. IIRC it's absorbed into their blood stream.
Of course if your tap goes through a RO system then it's detoxified and has already been dechlorinated.