I don't think dinging the tips is much of a consideration. They don't add much length to the tail pieces. And if anything, fiberglass is better able than aluminum to take a small impact without sustaining damage, so in a sense they can protect your tail pieces from minor mishaps (like a bumper on a car).
I think the biggest consideration in deciding when to do the tips is the mess and dust associated with fiberglass work. For that reason, some folks choose to wait and do most or all of their fiberglass work all at once, toward the end of the project.
In my experience, it's not a big deal one way or the other.