You really don't want to be touching the top tube with ANYTHING when you're straddling the bike (although there are others here who say that's BS -- personally, I'd prefer not to). Old wisdom said that a fist between you and the top tube for MTB was proper, and 1-2" for road/cyclocross/hybrids. I'm of the opinion that, as long as you can slide off the saddle and not hurt yourself, you're good.