I tend to try and max (most) of their specializations when they are unlocked, which probably means using the respec amulet for at least my usual party. But that also somewhat depends on if FF is on of off, because if I want to be lazy with it on then for the mages I get more mileage with a mixture of elemental trees, although I will still max their specs. For Sera and Varric, they get nearly everything in their passives either way. Also get most of Templar and Champion either way, of course.
Cole, Vivienne and Iron Bull are my least used squadmates. I did spec Bull fully into Reaver as an experiment to see if the AI could play him decently, but it didn't really seem like it was working out, at least not with the way I setup the tactics. If I were to try again, I might just put Devour / Consume on active and grab most of the passives, then fill out the rest from 2H and a mix of Vanguard and Battlemaster. AI does seem to do ok with Charging Bull now.