I haven't had to use a rack with my Tarmac. But I see no reason not to use the same rack I did with my Paramount that simply hangs it on it's top tube.
If you have a bike that you can squeeze the tubes between your fingers and feel some give, then maybe they need special care. Are there any bikes with tubes like that?
And how far are you going with these bikes in the rack? A few miles from home or a trip from coast to coast on who knows what road conditions?