is it safe to use real roses on a fondant covered cake?
If you can be absolutely sure that no chemicals have been used on them, yes. Just be sure that any wiring is properly encased in a food safe barrier.
If you can not be sure that they were grown with absolutely no pesticides then just use the floral vials in the cake section of Michaels or in your local cake supply store. With fresh flowers it's very important they never touch the cake...some types of flowers are actually poisonous! YIKES! So better safe than sorry.