you shouldn't cross-gear the bike (small chainrings and small cogs), but I'm sitting here with a pretty expensive bike cross-geared on the stand, and it's not rubbing the middle chainring. You shouldn't do this, because it does wear the drive train excessively. The middle chainring should work with pretty much all of the cogs, maybe not the biggest or smallest depending on the setup, but you should be able to use it with most of them.