As long as you don't use any type of chemical fertilizer on your roses, and you don't spray them with any sort of bug spray, or any type of spray to treat your roses for black spot or other types of diseases that rose bushes sometimes get, then feeding your beardie rose petals is completely safe. Just be sure to rinse them with cold water from your faucet before feeding them to your beardie.
Of course, they should not be fed to your beardie on a daily basis, but once in a while, as a treat, most beardies do enjoy them. My female, Eden would eat all the petals off half the roses on a bush, if I let her! LOL
Although, as a general rule, its not a good idea to feed your beardies plants from your backyard, due to the fact that they may be contaminated by chemicals or pesticides, or chemicals from exhaust fumes, if you live near a major highway or a busy street, with lots of traffic. However, if you don't use such things on your property, and you're absolutely sure that none of your neighbors do either, then a little treat from outside, once in a while certainly won't hurt them.
Other plants which they enjoy, and are safe to feed, occasionally, as long as you're sure they aren't contaminated with anything, are dandelion greens and flowers, the white flowers on clover, hibiscus flowers, and flowers from a Rose of Sharon Tree or Bush, since that is in the hibiscus family.
However, these treats should be fed only occasionally, and make sure to wash every thing from outside thoroughly, in cold water, and check to be sure that there are no insects on the plant before offering it to your beardie, since any sort of insects may have been exposed to pesticides before ending up on the plants in your yard.