Generally tweaks on actual personal meshes I dont as such. When Ive rigged my own stuff I know its limits and because Im doing stills and not animation I can get in there and simply paint it out in Zbrush or even in PS after the renders have popped.
But When it comes down to rigs and stuff in production, theres a zillion and one things that are going on, theres blendshapes, muscles, cloth, and even displacements etc......plus theres more than one dude involved in all the processes and everyone has their say. Generally the model meshes are fine but whether they fit a camera is another matter. Sometimes refernces can be misleading as opposed to a final shot
At some stage any of this stuff can just explode in a rig depending how far they are pushed....its not all perfect. So sometimes in specific areas something may not work - period, so its necessary for any team to evaluate the entire thing. This means start with the modeller and finishing at the final comp, its a matter of whittling it down until the prob is found and then you tweak it...then go again and so on until it works.
hope that helps