Definitely balance all six. From personal experience I can tell you you'll have longer tire life and a better ride if you do.
We had the front tires on our 2001 Adventurer replaced and balanced with Equal powder. They lasted about 60,000 miles before they had to be replaced a second time.
A while later we had the 4 rear tires replaced with tires identical to the newer fronts and also "supposedly" balanced with Equal. With a little over 20,000 miles on the new rear tires I could feel a slight imbalance problem from about 45 mph to 55 mph.
By the time the tires had 30,000 miles on them the vibration and balance problem became extremely irritating. We finally stopped at a reputable tire store to have the balance powder removed from the rear tires and balance them with external weights. When they started the process they called me back to the shop for an inspection.
IT SEEMS THE ORIGINAL SELLING AND INSTALLING DEALER DID NOT INSTALL THE BALANCE POWDER AS THEY WERE PAID TO DO.
The tires had gone 30,000 miles without being balanced. They now had large flat spots. No matter what balancing method would be attempted now would have limited results. The tire store did attempt to balance the tires with external weights. The worst one took over 12 ounces of weight while the others took between 8 ounces and 10 ounces.
Even with all the weight added the tires still couldn't be balanced to the point there was no vibration. They had to be replaced within another 5,000 miles. Tires that should have gone 60,000 miles only went 35,000 miles because they had never been balanced