I agreed to have IV fluids during labor when I started throwing up and was afraid I'd get dehydrated.
Today, I saw this study suggesting that getting IV fluids during labor might make the baby's weight read higher than it really is. In turn, this can cause the baby to seem like he/she has lost too much weight and, for breastfeeding mothers, can cause doctors to suggest supplementation when it's not really needed.
I don't know enough about the subject to have much of an informed opinion, but it sounds reasonable. I was so worried when my baby hadn't gained weight, but I was also confused because she'd been eating really well. It'll be interesting to see what comes of this finding.