Sono tubes are meant for vertical bearing. That means they are designed to be put in plumb, and carry weight straight down. For lateral strenth, which is what a post is, you don't want to use a sono tube. Same thing as a fence, no sono tubes.
A sono tube will carry a lot of weight, but a post in a sono tube can easily be rocked side to side, and will eventually fall over if its not tied into something solid.
For your purpose, no sono tubes, 8" holes (assuming 6x6 posts), and fill with concrete. Dig the holes 4 ft, trust me on this. I've seen 3 ft deep holes push a post out a foot! Done deal.
S.