I really like using a water soluble flux to assemble everything that is washable. I don't use an ultrasonic cleaner yet, but a good soak in 60C water with detergent, followed by a lot of hot water rinsing, final rinse in distilled/di water with a some isopropanol as a wetting agent, then forced air drying.
I'll go back and solder the non-washable parts with a no-clean or conventional rosin flux.
I don't especially like the idea of using a non-water-soluble flux and then just solvent cleaning by hand, I think it just smears it all over, probably negating the value of that step. (Sure, a whole tank of solvent or better yet, vapor phase cleaning would be good.) Some day I should make a test board and do some electrometer measurements to put some science to my opinions.