I'm going to disagree with what rickygsg has said. You don't have to worry about your surface being level, but it MUST me flat. You don't have to be building museum quality to worry about a flat surface, you need to worry about it for any model. If your surface isn't flat you can build a warp into you wing and fuselage structures. rickygsg, I'm glad that you haven't had any problems, but you've been lucky. I've seen what can happen when wing is build on a surface that's not flat, and it wasn't pretty. The wing had so much warp built into it that we couldn't straighten it, but rather the owner had to trash the wing and build a replacement.

If you care about the plane that you are building, please make sure that the surface is flat. It's fairly easy to check. Take a 3'-4' straightedge and place it with the straightedge down on the surface of your building table. You will be able to see if the surface isn't flat against the straightedge. Check in several different directions.