Querying etiquette?

So today I attended a writing seminar, or at least part of it... namely the section on "getting your work out to other people", and naturally a part of that was talking about agents.

A question came up about sending to multiple agents, and the response was in no uncertain terms, NO.  We were told that sending to multiple agents was a huge faux pas, not to be done, a mark of unprofessionalism, etc., etc.

Now, this didn't sound right to me, but I held my tongue.

So I put it out here: unless an agent asks for exclusivity, they don't expect it, right?   A query isn't a commitment, it's... just a query.  You can send them out to five or ten or a hundred agents at once (smartly, of course... no "Dear Insert Agent Name Here"), and it's not a big deal.

Or am I wrong?  I'm perfectly willing to admit I could be.  I'd love some insight here.

Querying in the summertime?

Is July typically a month in which agents take vacation?

I just send out another round of queries last week, and, oddly, I haven't heard any responses yet.  I mean, many agents take several weeks to respond, of course, that's normal.  But typically, in any batch of ten or so, there are a couple who turn it around in 48 hours, though usually with a rejection.

Not that I'm in a big hurry to get rejections, mind you.

Of the queries I have out there, there are three who are currently closed to new queries; a planned, temporary closure from July to September in all three cases.  I find this fascinating.  Is summer the unusually heavy time for querying?  Or is it that three months of closure is necessary to be able to take a mere two weeks of vacation?