Is it considered rude to enroll at a funded conference and then retract your enrollment?
The negative effect is that it may be difficult for the organizers to re-allocate the partial funding they'd allocated to you, so someone else missed the chance for funding to attend.

On the other hand, "the system" is not really actively worrying about your larger welfare, nor about the welfare of whoever missed funding due to your change-of-heart, and "the system" will not be "harmed" by your withdrawal. The organizers will scarcely notice whether you were there or not, funded or not. It's small potatoes. 

Between those two extremes, granting that you yourself do have to reasonably play the odds for funded attendance at conferences, you should aim to be "a good sport", not inducing commitment of far more resources than you'd possibly use, thus, freezing them and denying use to others. But don't be exaggeratedly altruistic toward "the system", since the relationship is very unsymmetrical, not in your favor.