Never query a literary agent until your manuscript is polished, perfect, and ready to go. Don't jump the gun. Don't be in a great hurry to submit.
Sure I'm always being rejected so what could it hurt to test out a new query for the manuscript that was finished two months ago? Query, and then do the final polish, or that was the plan. The first chapter's done, and there's time to do the other twenty-three. Three hundreds pages of words, how long could that take?
Don't try this at home. I sent out three queries yesterday morning, and had three replies in a matter of hours. Two rejections, as expected, and a request for a full. To be sent via e-mail.
No stalling, no playing for time, not with the speed of e-mail. No need to save up for a ream of paper and the cost of the postage, no indeed. Touch a key, hit a button, and the manuscript's away, but it's not ready to be sent yet is it?
In between doing such things as, oh, say, earning a living, I have to polish the manuscript, slice out the extra words and make sure it's all clear. The longest I can stall is probably until Sunday night.
Trust me. Don't do this. You get one chance with an agent who asks for your manuscript, and you don't want to rush into submitting when you could blow that chance.