I write software as the largest part of my business, and I don't think I've ever been asked to assign copyright to the purchaser. I go out of my way to point out in written materials that I use licenses like GPL, BSD, and put stuff in the public domain when that's compatible with library licenses and such, but no client has ever actually asked me about that.
In my experience, people who are buying custom software don't even think about copyrights and licenses; it just never occurs to them.
