Its beyond tacky, its wrong; your taking advantage of the nature of your relationship with these people so you can get some link pop. Here's the thing, these “clients” your slapping your link on do not have an adequate understanding of the ramifications of having that link at the bottom of their home page, and often on the bottom of every single page on their site. If they did, they would not have hired you to create, or to optimize their site. If they hired you to “optimize” their site how exactly is this link of yours helping that effort? They look to you as the trusted authority, and as such are vulnerable to being steered into decisions that are not in their absolute best interest; even if they did “offer” for you to place it there.
That link is helping you more than them, however little that may be.