Josh, you can actually open that valve and end up waterlogging the whole system. You only want to add water when it is too low. If the boiler has too much water in it, then there is no room for water to drain back down.
go to www.heatinghelp.com and look at Dan Holihan's site. There are some excellent articles that can tell you everything you wanted to know about your system. They also have a forum there that deals with boiler systems.