I have always used mode C but was told on the weekend I should select A.
I'm curious who told you that and why. Your profile says CA; in the U.S., a lot of verticle manuvering will cause our ATC computer to disregard your Mode-C and show
XXX instead of your actual altitude. Still, any time you're not exceeding around 3000 fpm, we should still be able to read it. I'd prefer you have it on in my airspace.