We number our prints but we do it on the back so the front can stay nice and clean. We started out just stamping our shit on the back and did that for about the first 6 months of posters. After that we started signing and numbering underneath the stamp. Neither of us started doing it for collectable value because that just doesn't exist for us. But when you're actually printing your own posters you want some way to signify that, yes, this poster was hand printed. At least that's why we do it. So when we say "these are hand printed" and they say "oh yeah?" we can flip the poster over and show them that they're all stamped and signed and numbered. In a way it's kind of proof for people.
I agree with Art's assessment of not signing but I also know that Art isn't printing his own posters. I think if someone is not only designing the poster but spending hours sweating over an inky squeegee to get some posters done they have a right to sign that piece of shit. It's just as much art as any art print as far as the process goes, it just has a purpose.