I don't think it really matters that much. If you have doubts about your sanitation, then go ahead and boil. If you sanitize good, boiling might not be necessary.
When you boil an extract, it has a tendency to make the wort a darker color. If thats what you want go ahead and boil. If you're going for lighter/clear color maybe you don't boil.
If you're making a kit, its pretty hard to mess it up anyway.
Personally I wouldn't boil it, but thats just me.