I'd say the most likely causes of death for 10th-level PCs are:
1. Being attacked by a monster while unconscious (either to prevent healing or because the monster is just spiteful).
2. Getting caught in the blast of an area attack while unconscious.
3. TPK.
It's taboo. So no, you shouldn't do it.
What, it's taboo because you say so?
Maybe your table prefers not to do it, and that's fine, you play however works for you. At other tables, however, the players
want the DM to pull no punches, and in that case you absolutely should do it.
My own attitude--as a player and as a DM--is that the monsters should fight to win, using all the smarts and tactics available to them. Normally, monsters don't attack downed PCs for the same reason PCs don't attack downed monsters: In-combat healing is quite rare, so usually it's a waste of time to be slitting throats instead of concentrating on active threats. However, intelligent monsters who see clerical healing going on will take steps to prevent it. That includes a) targeting the cleric, and b) finishing off downed foes.
Of course, not all monsters are cool, rational tacticians. A monster of animal intelligence won't ever "figure out" healing magic; it'll ignore downed foes even if they have repeatedly bounced back up. Conversely, some monsters are just
mean. They'll go for downed enemies even when it's not tactically advantageous, out of hunger or sheer spite. Ghouls and gnolls are good examples.
Fiinally, some monsters can't tell when foes drop unconscious, and will just keep noshing on 'em. A swarm is the most likely example. (Of course, it can't tell when the foe is dead, either. Putting your pack animals in front of you is a good way to distract swarms.)