One argument, unless you are specifically trying to drive a downstream plugin at a higher level or something (which for the most part isn't really interesting in the plugin world anyway), is that you should adjust each plugin's output so that it sounds as close as possible to the same perceived level if it's in or out. The reason being that it allows you to do good A/B comparisons of how its affecting the sound. If the direct vs. processed sound is at all uneven, it will biase your comparison pretty heavily.
That also of course means you can remove it at any time without whacking anything downstream.
As long as you recorded at reasonable levels (not too high, not too low), then the natural level of the tracks should be generally fine to feed through the plugin chain, then just use the fader to adjust for downstream summing needs. Though some plugins these days may try to emulate being driven harder, but probably most don't and so it doesn't much matter as long as the levels are reasonable. Some of them may emulate a device that has no adjustable threshold, but they'll also have an input gain so you don't need the upstream plugin's output to do the driving into the fixed threshold.
Then again, you could just run everything wide open until it clips horribly and create a Dubstep hit by accident, who knows...