As a studio, we're opposed to credits within the actual design. However, we do generally include our name and URL in both the meta data and stylesheets.

No. It devalues your work and the client's brand. Perhaps try something more subtle like a note in the markup.

Yep, when it's made with <3...

Personally, I think it's tacky.

humans.txt

wow, i never thought that it was unpopular to do this. The way i've found most of the good design agencies I know about was through a link on the site.

I personally consider it okay to sign your work if the project went well. It says "hey, I think we did a good job here, enough that I am willing to put my name on it!" But if a Client does not want it there then i am more then glad to keep it out.

If a site has an imprint then I find that that is often a better place to put it, as it really isn't site content, and maybe doesn't belong on the site footer.

Definitely in the source code on every page, the nerds who look at that need to have a place to complain about the stuff i did wrong!

Not unless working for free, and usually not even then.

Nope, unless the client actually asks for a watermark for some reason.

I generally leave my information only in a HTML comment and in meta tags, but if asked or if it's free work, I also sign on the footer.

Never. But I sign the uncompiled code itself (never shows to the final user after compilation).

I usally put a simple "Website by [Me]" at the bottom.