I get about 80% of my design and
SEO work via those small links on the bottoms of sites. Some are more relevant than others of course, many are not relevant. They do not give me as much benefit
SEO-wise but they do give some.
I always do it with the clients consent, if they don't want the link their I'll remove it (extremely rare I think I've only had about 5 clients sites out of about 800 where I haven't linked back).
I do the links site wide as I add them in the footer and I use SSI for almost all my sites.
When a client leaves me or the site is redesigned I always remove or edit the link.
Also I make sure the link text is unobtrusive and specific to the job.. ie. web design by: orionsweb.net or hosted by owhosting.com etc. sometimes I don't link them and just put the text also.