I have a mountain bike that I use most regularly on roads and packed dirt trails. As such, I replaced the knobbie tires that came with the bike with a more street oriented (slicker) tread style. I currently have my knobbie tires in the closet. I intend on switching the tires back to the knobbies whenever I do any true mountain biking or other activity that requires the more agressive tread.

My friend does the same thing (as far as using slicks in the city and knobbies in the country). He took things a step further by getting himself an extra set of rims and another cassette. He then put the knobbies on the spare wheels. This way, he need only use the quick release to change tires ... as opposed to letting the air out of the tube, removing the slick tire, putting on the knobbie, etc.

I heard a rumor though that this method of using two wheels is a bad practice. That a chain and a cassette wear themself out together, forming similar wear patterns. As such, when I put on the cassette that has been on the wheel in the closet ... it won't fit right ... sorta like loaning your buddy with a different shape foot your broken in sneaker.

I have no real need for making super-quick changes (like a race senerio) where two sets of rims are the only way to go. As such, should I stick to the dirty job of removing/putting on tires each time I want to change the tread. Is the rumor above true?