However tightly you set this nut will determine how loose your skateboard trucks ride. Some people like stiff trucks, so that they don't turn unless you really put some effort into them. This is good for lots of tricks, because your trucks won't flex too much when you land a trick. On the other hand, some skaters like loose trucks, so that they can carve better. It's all up to you. You can tighten and loosen your trucks any time you want.
If you are performing maintenance on your skateboard, you might want to take a look a the Skateboard Maintenance area for some more ideas and instructions. Perhaps for cleaning your skateboard bearings, or applying new grip tape. It's all there - have fun, and skate on!