SAM is the only melee DPS that does not have extra initial potency in positionals, but it only has two that it needs to hit. Instead it unlocks a trait where it gains its gauge - Kenki, from hitting Gekko (back) and Kasha (side).

If you've just unlocked it and you're at around 50-ish, then don't worry about it until you unlock the trait. The difference positionals make on SAM is pretty slim, so if you don't want to hit them, then you don't really have to.