I also vote no.
I will admit, sometimes, in the past, I've forgotten to mark things as installed but, that makes it so much harder for me, when I'm trying to find support on it or whatever. So now, I'm in the habit of tagging mods I want to look at later, or have downloaded to try. If it works, and is what I want, I then mark it as installed. But it may be a week or two later when I go back to it, usually mark several of the tagged mods as installed at once.
If I had to click installed before I downloaded it, I'd find myself having to mark it as uninstalled if i wasnt installing straight away, and then marking it as installed once i do install it, if it works, seems, too much hassle, especially if you're after a few mods at once to try out. Like upon upgrading to 4.0 i'll be downloading many of my tagged mods but may not use all of them, or not what I want.
I've also, often downloaded the file more than once

. If I can't find it/remember where I put it, or am on a different computer and want something from the read me or whatever, I'll redownload.