I did it to get into a bar that books bands 6-8 times a year. I knew the bar owner from before and he was willing to give my new band a chance. I didn't like doing it, but I figured it would probably pay off in the future. And since I have played there steadily over nearly three years now with my current and former bands, I would say in this case, it paid off.
I wouldn't do it unless I was pretty sure we would get rebooked several more times in the future. If it's for an out-of-town club that may just turn into a one-nighter, don't do it. Quote your full price. I gave one club a $100 discount when they asked "How much for two nights?" after I told them the one night price. Obviously, this is their way of seeing if you will 'work with them' because it's damn easy to add the original price times two, right? After giving them the discount price, it seemed to just give them the idea that we had to go even lower if we wanted to work with them in the future. We don't play there anymore. I'd rather play places that just pay what you expect than have to fight, bitch and moan with people over it.
Stick to your guns, but if you want to try the discount, as RupertB said, be ready to walk away if they refuse to go up to your non-discounted price in the future.