I use to spin balance everything. After trying the Dnyabeads on my last coach, I'm now a believer in using the balance beads. Spin balancing doesn't account for tire wear unless you spin them every 10K.
I also like Centramatic balancers. You have to plan ahead and order them before you do a tire change. The extra cost on top of the tires can be tough. I just bought them for the front of my new coach ($168.00 pair). I had a local tire shop that I do business with install them. Since I was a regular customer, they just had me tip the workers.
ALWAYS do all of the tires. Even if you can't feel it, you could end up with uneven wear.