In 3x, very rarely. I thought was easier to build from scratch than it was to modify an existing monster.
Same here. In 3e, powering a monster up is fairly straightforward, but scaling one back is something of a pain to do "correctly", imxp. So I usually take more to a bottom-up approach:
(1) define a monster "kind" (eg, incorporeal undead),
(2) identify the most common salient features of that "kind" (eg, ability damage, miss-chance, spawn)
(3) devise a scale of increasing power-levels for those features/abilties (eg, 0, 1, 1d4, 1d6... ability damage).
This-- along with a few skill packages, relevant feats, and more specialized monster abilities-- provides a nice pile of informal "building blocks" that easily fit together to make fairly generic versions of the monster "kind" in question.
As a procedure, this isn't really as formalized as I've made it sound here, nor is it "correct" or truly balanced by any stretch of the imagination. But it works well enough to throw together a run-of-the-mill mook to fill a thematic gap when no MM creature really works.