Canter elevates a lot of units in terms of positioning. Basically everyone who is earning EXP/SP on player phase can do well with Canter. The only actions that don't activate Canter are those which don't earn EXP/SP... such as Wait, Repositon, using an item, Instruct, Dragon Vein.
Technically you don't need Canter to have your unit be good. Specially if you have specific plans for a unit that don't involve them player phasing or maybe you want a more immediate boost even then; Alfred for e.g. only activates his ability if he chooses Wait, and so if you are looking to use him longterm and take advantage of his ability in some way, then you may want to consider something else. Clanne is another e.g., a Mage unit who has pretty dodgy magic growth but great speed growth, so maybe you would prefer Mag+2 instead. Even then though, Canter is still there to help for most other player phase actions, so it's hardly ever a bad investment.
Like I said in that post, I'd say definitely consider Canter at least for your units that get danced the most; let's say about 6~7 of them, at the very least. Including Alear and Seadall in that too. It makes setting up Goddess Dance a lot less cumbersome. Even Micaiah staff healing is easier too, even if you don't like skipping strategies there's net benefit there too.