It’s entirely dependant on whether the client will allow it. If they have paid for the design, then legally they own it and are perfectly within their rights to not allow you to put your name on there. Generally speaking, the larger the company, the less keen they are to have a reference to somebody else on their site.
Having said that, if you can get away with it then by all means tuck away a discreet link somewhere.
I have stopped doing this recently. For SEO, you end up dragging down the website you add a link to ever page of their site. It ends up leaking a lot of their page rank to you. I care for my clients’ rankings so I just put one link from the about page somewhere… My largest clients, I don’t even put a link back.
If they have paid for the design, then legally they own it and are perfectly within their rights to not allow you to put your name on there.
Apparently this is not always the case. In Montana for example, the courts have seemed to lean in the developer’s favor: ie The developer created it, and still has rights to it – like a painter who displays her work in her online portfolio even though someone bought the painting already. (This is word of mouth from another Montana Developer who’s been doing it for 20+ years. so don’t forget the salt.)