Is it safe to unplug a drive while MacBook is asleep? (i.e., does sleep unmount drives?)
No. Logging out dismounts the drives.
USB changes will wake most macs from sleep assuring an uncontrolled unmount should users be logged in. The file systems should be clean and synced when entering sleep, but there is no guarantee of it.
Firewire drives also could be corrupted in a similar fashion.
With journaled filesystems you'll likely only lose a few files, but it's not the best practice. Plugging things in as fast as possible is the best option in case you can get things back before the system wakes.