It's usually been the case that the scalers in TV's have not been as good as those found in players, depending on how cheap the player is but that's been the norm as far as I was aware, the UB900 you should definitely use its scaler as it's very good.
I suppose it depends on the range / price of the tv though?
While I appreciate the player is dedicated to one thing rather than the TV that has to cater for everything from analogue to streaming, and other sources - a £3k or £4k tv should have a better scaler than a £500 player