devkitPro

Title.

This is generally good practice, especially if you want your code to work on multiple platforms. It's quite a vague question though. Was there something specific you wanted to do.

No. I just wanted to know if it's bad not to free for example the C2D_SpriteSheet on app end.
Or in other words are the resources used by my app automatically cleared after closing or are the resources still reserved?

While freeing heap resources on exit generally doesn't matter, is there any particular reason why you're trying to figure out if it's ok to avoid freeing resources? Could you post code, so that we can understand where you're coming from a bit better?