Yes! Medspam is god! Like mrben said, don't just look for the med icons, watch your marines.
Random thoughts:
If a skulk is all over a light marine w/lmg biting him, don't drop meds. Chances are he's out of ammo and will die within the next few bites anyway. The meds would probably throw him off anyway. Calmly wait for him to kill the skulk, THEN drop meds. Just thought of this randomly, anyone agree?
(Totally Off Topic)
Lets say you have a phase gate up to the hive, you drop 5 shotties, and want them to run through and rush the hive. Rather than just say "DON'T HUMP ARMORY PHASE!", drop a whole crap load of ammo packs around the armory and phase gate. That way, the players that ignore your rushing plee's will get all the ammo they need and phase right away. Plus, it'll be a pretty good hint to them that you're serious.