Writing can be a kind of "hurry up and wait" situation, with a good amount of time between submitting a manuscript and actually seeing it in print. My first book (un-agented, but with a traditional publisher) is due out in Spring 2008. I'd like to start querying agents for my next "big project" but would like your advice on whether I should wait and see what the reception is to the first book (it's a history title) or begin querying in advance.
This is a tough one, and I expect you would get different advice from different agents.
There's no special cachet that might make me look at your query differently if you waited to pitch your book till after it has been published, excepting possible sales and publicity. But this is a double-edged sword - your book may not sell well (no matter how well-written it is), but if you don't wait for publication you could miss an opportunity to cash in on any possible success.
I'd probably suggest querying agents now, unless you're expecting some good reviews or blurbs to come in for the book in the near future, or the publisher intends to spend significant amounts of money on publicity. Also, the querying process is long and ongoing, so it's quite possible that you could still be querying by the time your book has published and you can then alter your query accordingly.