I would add to the excellent advice above that to paint the town with submissions leads to the overwhelm editors experience. In turn, this clogs up the submissions system and it takes more time for me to get a reply.

If I know an editor to whom I am submitting, I wait to hear back before sending to someone else; or check back within a reasonable time period.

Same goes for if I am using someone's name as referring me.

If over the transom, I send 5-10 queries at a time for a literary submission, and 10-20 for a commercial. As soon as I hit "send," I make a list of the next 5-10 or 10-20, and send out as rejections come in. 

Thanks, Dorian and Ellen.

I agree.  In your cover letter, let each publication that accepts simultaneous submissions know that you are submitting to more than one.  Then, when you have an acceptance, let the others know as soon as possible. 

I think it depends on the publication. Each one should have submissions or writer's guidelines which should lay out whether they accept simultaneous submissions or not. You'd want to check them out anyway before sending anything out.