My mountain bike (street beater now) is a 94 Specialized RockHopper Sport, 7 cogs on the back, 3 on the front. Mine rubs quiet a bit on the big/big combo but thats it. All other gears are accessible with no rub. Derailers are Shimano "STX". This hard tail has been ridden hard off road, jumped, raced a couple times, wrecked, wet, dirty, muddy, etc. I've cleaned it but never had to adjust the derailers at all.

I believe Specialized has some pretty extensive experience in the triple ring arena. I'd say your LBS is full of BS.