I don't think a trim plug is using much RAM. Even if it was if you're only using one trim plug as the last plugin in the chain as opposed to the first plugin in the chain the the memory usage is the same.
Another problem with setting the level with the last plugin, what if you add another plug, change it for something else or even just change the settings? You have to change the gain in multiple places. That's not the reason why I do it this way though, I do it this way mainly because to my ears most plugs seem to sound better working in the realm that the manufacture presets sit which is usually high in high out.