Is it considered rude to enroll at a funded conference and then retract your enrollment? For background, I am a mathematics graduate student.

Now, it happens once in a while that I find an interesting conference/workshop which I would be happy to attend, but which I'm not sure about (for reasons which might appear later on, such as not enough funding, finding a better conference etc.).

Suppose that in case of acceptance, the organizers offer some travel/accommodation funding, and that enrollment closes a relatively long while before the conference takes place. Also, suppose that I am not offering to give a talk.

Is it inappropriate to enroll with the knowledge that there is a relatively big chance I won't be able to participate, and retract my enrollment after being accepted if I realize that I indeed can't go?

Does the answer change if I enroll at several conferences "just in case" and then go to one of them if I'm offered enough funding to cover my expenses?

Is there a point after which cancelling my participation is considered rude? What is the polite way to do it? Is it common, in your experience, that (student) participants cancel their participation after having been accepted?

Thanks!
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.