A question about itemization on non mana champions

I understand that certain champions do not use mana and therefore you shouldn't really build mana on them because it's a wasted stat that you pay for (Gold inefficient). My question though is whether or not you should buy an item that has mana even though you can't use the mana, but it has a passive that is pretty good all the same. Such as getting a frozen heart on Garen or buying the new zekes item on shen support. Is it efficient to buy the item even though the mana is wasted just for the passive and the other stats you can use as well or is it just best not to use the item at all because you don't use mana?