My personal rule is simple. I never ask for gas money for local trips. But I only offer trips to where I am already going. So my friends get to enjoy the convenience of a free trip and I never go out of my way as a free uber. How far out of your way you are willing to go to pick someone up is up to you but generally (not that it's codified) I'll go up to 10 minutes out of the way to pick someone up.

What often happens is I'll end up dropping off (or picking up) people along the way at a metro stop