I bury the bud union about 3 to 4 inches below the ground and never have any problem with suckers.

I have my bud unions, all multiflora, buried between 3-6" and I have never had a sucker on any rose, ever.

I plant my bud union just below the surface, then the rose goes own root. Suckers tend to come from the ones that are not planted deep enough.

What they told you at the valley nursery is absolutely positively true for every other type of plant at the nursery except roses and tomatoes. Those you can plant deeper without any problems. Most other plants would die of crown rot from the same treatment. Roses and tomatoes are the exceptions.

I would bury the bud union just barely. I don't think I would plant quite as deep as people in real zone 5. Even at 6000 feet you are not going to have the brutal repeated freeze/thaw cycles that the midwest gets or serious below zero cold for weeks and weeks like Maine or Vermont. The roots of the plants will do better with more oxygen, and there is more oxygen towards the surface.

Clematis is one other genus you can plant deeper than it was when you got it. Forgot that one!