The quick and dirty answer is "It depends".
Sorry. Some motherboards use the mounting screws as a part of the M/B gounding system in which case you DON'T want insulators (rubber or plastic washers) there. On other M/Bs the ground system is seperate and connects only through the power cables and you DO want the insulators.
I'd assemble the system with them in and see what happens. If everything works fine just leave it then. If not, disassemble and take the insulators out. *shrugs*