If you're going to drop the weights, the only weights that touch should be bumpers, and you should be on an Olympic lifting platform of some kind.
Bumper plates of different weights have a consistent diameter, to maximize surface area. Additional iron plates can be used if they are of a smaller diameter.
Make sure you're using a quality bar. Ivanko, a quality manufacturers of bars, has an article (PDF) with some gory stories about poorly made bars snapping in half in the middle of a lift or on a bounce.
You should try to minimize the amount of bar-dropping that you do, but sometimes you can't avoid it. If you're training the Olympic lifts seriously, you risk a missed or dropped lift. That's the reality of lifting close to your limit. But for most warm-ups you should be controlling the bar for most or all of its downward path. (This is more true for cleans than snatches.) Many people ignore this advice and drop every attempt in the attempt to minimize the eccentric, soreness-producing phase of the lift. In some gyms this is considered rude and destructive. Either change gyms or change your behavior.