You can of course 'try before you buy' by implementing Sandbox on your blog and saving a copy of the page to your computer. Granted, this suggestion is not much use to those who don't already know their way around a stylesheet, and I suspect that group has made their decisions by now.
I can't even be bothered to make a custom header for wank.wordpress.com, so forking out for custom CSS seems unlikely unless I decide I am in desperate need of a .com testing ground.