With DS he dropped 2x. The 2nd time he dropped it was around 30 weeks or so, and if i remember correctly my OB said he was in place (ie: engaged) but that didn't mean that i would go into preterm labor. I felt him sitting there on my cervix with every step i took (yeah, not too comfortable!) but he waited until his due date and came on his own then.
If you're really worried, just ask your OB if he's concerned about baby being so low. Never hurts to ask.