Honestly I'd be fine with that if they admitted it. I have way more respect for someone who says "I don't know" than someone who doesn't know but tries to pretend they do.

I've had good experience at ACE, but they are typically smaller so I get less selection if they even have what I want, and in my area they tend to be more expensive. I trend towards Lowe's because the staff will give me the "I don't know" answer.