I think that talking to your doctor about it in advance is absolutely the thing to do. Ask if they feel it's necessary for you to have, and why. If it's just a routine deal, then just say you'd like to decline it. If things are going normally, then there is no reason to have a routine IV. When you go in when you are in labor, tell the staff you've talked about it with your Dr. and he/she agreed that you did not have to have one. Then they can call the Dr. with questions, which is much better--you don't make enemies of the nurses, which they really shouldn't be.
If they feel you DO need an IV, ask why. It may be that there is a reason, and then you can consider that. For example, if you'd had hemorrhages with your previous babies, or are at high risk for a hemorrhage for some reason, that's a good reason to have a saline lock. If it's just routine, and not needed, then great, say no. If it's to be able to give you drugs to stop bleeding that you are likely to need, just be aware of what the course of action would be if you don't have the IV, and what they would be able to do for you. It's not fair to expect the Dr. to be able to fix a problem if you have chosen to take away the tools they have to do that, you know? So ask why they think you need it, and what would be the consequences of not having it, and then decide what you want to do.
Annie