I do, yes. I figure doing so probably looks better than being able to answer the question (or rather, than just being able to answer the question, since you still answer it of course).
One time I got dinged for using a library to calculate standard deviation (as a part of larger solution) rather than doing the math by hand. I guess I assumed they were more interested in codecraft than in arithmetic and I was wrong.
Ah, that's pretty crappy. I try to avoid that sort of thing by prefacing all library calls / fancy language features with a "I'm going to just use [this thing]. If you want me to go into that, I can". Not so much ask permission as tell them. It's really a pain in the ass to have to be on guard like that though.
