rainborick - 7:22 pm on Apr 2, 2013 (gmt 0)
There are several ways to implement push buttons, and none of them should add any significant overhead to load times unless you have a large number of buttons on the page that each require different images. Even then, you can mitigate the overhead by relying on CSS sprites. And if you create the button with pure CSS, there's no overhead to speak of at all.
I can't say that I see this done very often, but there's no argument to avoid them that I know of. If the client wants it, I'd just do it.