You're not obligated to do anything and your friends should not expect rides from you. You're free to say no and not feel guilty. The beauty of public trans is that no one is actually reliant on you to get around - you're just a more convenient option.

I own a car and my approach is, if convenient (eg not more than a few min out of my way), I'll pick someone up or drop them off from their residence. What I do more often is bring people to a point where there's a mutually convenient parting of ways. This is most often a metro stop that is along my way and saves them some time/money/transfers. Unless there are extenuating circumstances (injury, significant inebriation, etc) I wouldn't drive someone across town and I don't feel like my friends expect it.