A couple of thoughts ....
Putting gravel in the bottom of a planting hole actually will impair drainage. You are making what is called a perched water table. This is fine for fast draining soils, but if you have adequate drainage with your native soil, it will slow the drainage a lot. When they build golf courses in hot, dry climates, a layer of gravel is always added before they put in the turf so that less water is required to maintain the turf.
I suggest you always do a perk test for your rose holes. If they drain in a few hours, you don't need to add gravel to the bottom of the soil. If the rose hole drains in about an hour, adding gravel is a good way to keep moisture in the root zone.
I only use loosened native soil in the bottom third of the planting hole, because the anchor roots are fed by the feeder roots located closer to the surface. As I move up, I mix in bagged soil that is used for raised beds ... it's more dense than potting soil. The closer I get to the top of the rose hole, I add more of the bagged soil to the native soil. It's important not to create "layers" of soil, so you just do this gradually. Any fertilizer goes on top after the rose has been planted before I put down my mulch and leaches down as the rose is watered.
Since my native soil has a lot of stones in it, using some of it mixed with the bagged soil, allows for those air pockets Mike mentioned. The roots need that oxygen.
This is my general planting method, but I also add a few things that my soil needs near the top of the rose hole for the feeder roots.
This method is what works for me in this garden because I am gardening in a hot / dry climate.
