I usually ask "how do I best do xyz with this library?" If I cannot find a good answer elsewhere online. Usually the developers answer and I am able to use their commentary to solve my problem. I know for one thing my answer will be public after I ask so at least it will become part of the selfdocumentation that arises from both stack overflow and issue systems.

Worse case they have to add something and put my request in some sort of backlog. I always say thanks when I ask someone for help though. But I never flat out ask for new features because for all I know they already have a defined way of solving the problem I have.