You should be in a low-ish gear, but you don't want to be geared so low that you can't get any momentum to help you balance as you start out. (I under-gear all the time on my mountain bike- it's not pretty)
There's a stop sign at the top of a steep short hill on my commute home. I'll often shift to a harder gear as I approach it, just so that I'll have some "kick" and balance when I start pedalling again.
If I'm approaching a stop sign, and I know I'll have to climb immediately after it, I'll shift to the "right" gear as I come to a stop. That way, I can take off when traffic is clear without juggling my bike.