Proper compaction of any backfill is the key to stability for any pier. That mostly means putting a couple inches of dirt in, ramming it down well (the end of a 2x4 works good), and repeating that, in 2" lifts,until the hole is full. That is mostly not possible with a sonotube-type form, it'll crush the tube....so wait at least a couple of days for the crete to harden before backfilling. A large hole dug by a backhoe can be quite a chore to backfill, I always try to dig any pier holes with a post-hole digger and/or small shovel, (or power auger) to minimize the disturbed soil I must put back.
You're doing the best possible by making sure all below the pier is well-compacted.
There are available at most real lumberyards one-piece tapered plastic forms that are stiff enough that they can be backfilled before pouring the crete, that is a nice option, because you are not working over an open hole.
Here's one I've used:
Edited by roscoe, 18 December 2018 - 07:52 AM.