I do as long as it isn't something you have to purchase. If it is that, I want to be paid for commercial advertising at that point. I think that is fair, too.
Yes i allow it on all my sites, I look at it as helping my fellow forum owners and admins to survive as forums are in the decline.
As other have replied here i will not allow links to porn and other garbage.
Given the niche that my forum is in, we definitely allow signature advertising. As for previous forums I've been involved with, I've always permitted signature advertising, so long as it isn't excessive. Users can advertise as they please with signatures, avatars, custom titles, etc.