Sammael
Adventurer
More and more often as the PC level increases. To do otherwise would be a suspension of disbelief for me. After all, high-level characters are more rare than their low-level counterparts. Right now, my 15th-16th level PCs are up against a powerful wizard and his four generals. One of the generals is a high-level hobgoblin monk/fighter/kensai/dervish, but the bulk of his army is still composed of regular hobgoblin warriors. Of course, I don't expect the party to fight them, but even the elite hobgoblins are level 6 fighters or so, and the PCs have little trouble dealing with them.