This is purely from my own views and experiences. Witchcraft is highly personal so if you feel something might not fit or you have other ideas then that’s perfectly fine!
Stems: wonderful with protection spells or to create protection talismans, especially if you keep the thorns on though it’s not necessary
Thorns: great for curses/hexes/general nastiness.
Petals: these are great because they are not just for love spells. You can use the different color correspondences for your different spells (I.e yellow for spells to help aid in building friendships.) on top of this you can use roses as a substitue for pretty much any other flower. They also can make a great potpourri if you’re not into all of that.
Stemens: I have yet to try this out but because of how flowers reproduce I would keep and use the stemens for firtility spells.
Rose Hips: not only do they have great medicinal purposes but they can be used in many different spells! Personally I would use them in something that has to do with new beginnings.
This is all I can think of, atm! I’ll probably add onto this as time goes on!