I would run man coverage on every play ideally... 5 men to cover the eligible receivers, rush 4, and use the remaining 2 defenders to 'help out' where needed. I even run it against 4WR snugs.
That said, I sometimes use zones when my opponent displays a penchant for breaking my base coverages down, inside the 10 where the zones collapse, or where a tendency presents an opportunity.
Of course I try to quickly adapt to what my opponent does, pinpoint what he's weakest against, and give him plenty of it.
Later