Is it OK to use washed, smooth gravel under a pier block footing?
Really Jay, that is the wrong stuff. I wouldn't use it alone.  It will not pack properly and will have a tendency to shift and let water rush through and undercut the surrounding soils.  You really need a good sandy gravel that will pack hard. I'm absolutely sure you will regret using washed round stone without a binder.  You could add Sacrete mix to it to firm it up.  That would help.  Mix the sacrete with the stones dry and just wet it down with a hose or a couple of gals of water, let it set up and continue. The stones will become the aggregate in the concrete mix and make an excellent base for you.