I've heard many opinions on this, and have yet to get a solid answer. Some writers send one query at a time, others send a few. My personal thought? It depends on where you're sending it.
I play by the rules with most things. I pay my bills on time, I haven't (severely) broken any traffic laws, I typically play well with others. But one thing I've discovered about myself is that I'm impatient! Especially if it's for something that I've set into motion, but I need to rely on someone else to give me the go-ahead (i.e. editors, hiring managers, etc.) If I have a story idea, I usually want to get the query written and sent right away--in my mind, I have to get to the editor first, before someone else pitches the same idea. Sometimes it pays off, sometimes not. Sometimes I get two good story ideas, so why should I wait to hear back on the first one if they might like the second one better? Doesn't it make sense to just send them both and cover all my bases?
I do sometimes send the story idea to two different publications. Again, most writers I know don't have the luxury of sitting around and waiting for the editor to make up their minds about using the piece. Perhaps the other guy will have a quicker turnaround and get back to me first. Of course, both could reject it, but in that case, find a third market, retool the query if need be, and send it out again.
I'm trying to find the fine line between patience and persistence. It's a fine one, I can assure you. Typically I give editors about a month before I start to bug them.