I'll do spells, but I really don't like to do them for other people. I encourage someone who asks me to do a spell "for them" to learn how, and do it themselves. I think spells work better when they are cast by the person for whom the work is being done. In my view, it's need based. When I'm in need, the energy of my emotional connexion to that particular need influences the spell, and makes it more likely that it will work. If I were to cast for someone else, I would feel less emotional investiture, therefore, less "Oomph" would go into it. I feel this would, at the very least, reduce the chance of success.
As far as curses go, Uh uh. Nope. No curses from this bird. What goes around comes around. Folk wisdom for everyone. I figure that if someone does something to me that would make me consider cursing them, they'll get their just desserts from the Universe anyway, so I don't have to do anything.
And besides, if it's something like an ego issue, where a percieved "attack" was nothing of the sort, letting the Universe take care of business keeps me out of trouble too.
Real evildoers, however, rapists, abusers, murderers, deserve whatever they get from the laws of Divine retribution.