My OB with #1 was in practice straight out of residency, and she says that she was taught to mag as soon as pressures are diagnostic 140/90, regardless of proteinuria or other symptoms. My OB for #2 has been in practice longer and says she doesn't mag mild PE (140s/90s, 300mg protein) unless there is hyperreflexia, headache, visual disturbances, or bad labs. It is *so* variable among doctors, it really seems up to their comfort level - and how long since they've had a woman progress to eclampsia...
I'm one of the ones who has been very vocal about how much I've hated mag [;)]. I puked, felt generally nauseous, couldn't lift my head, got very fuzzy mentally, AND the nurses didn't give it to me until I was 8cm so baby got the full dose and was floppy and had trouble breathing and spent 4 days in the NICU for high mag (MgSO4) levels. BUT, it is a lifesaver if you really need it.
I'd ask your doc what her / his threshold for mag is, just so you can be a little prepared. If seeing the baby right away is important to you, (and assuming you're lucky enough to have a vaginal delivery [:)]) ask for the baby to be put on your belly while your husband cuts the cord - that way you get a few seconds with the baby even if they need to do more to make sure the baby is healthy.
Of course, if there's a huge emergency, the baby's health absolutely comes first! I'm just sad that my doctor took the time to let my husband cut the cord, but I didn't get to see or touch him until he was over 30 hours old [:(]. If she'd at least put him on my belly I could have touched him... I know I'm supremely lucky that he was healthy at all - many women here have lost their babies and that breaks my heart. But even though my pain is so minimal in comparison, it's still painful for me, and I'd like anyone else to be spared the same experience.
Oh! and if you are magged, be prepared that you'll be on it for at least 24 hours after delivery. That's why I didn't see the baby - my nurse wouldn't take my gurney into the NICU. I found out later that some nurses would have taken me, it's just a pain in my hospital.
Sorry for the rambling book, here [:)]. Lessons from my birth: try to be prepared, and as long as there is a way to do it with mom and baby's safety in mind, at least *ask* for what is important to you in the birth of your child!