Usually at the beginning stage I'll stick to just one but divide it up into layer groups. I tend to try to one overall texture because it helps co-ordinate the colour/bump maps

I quote like the way you have colour only 'sub-panels' on your models though, I might look at something similar for this one. You wouldn't believe how frustrating it was to keep flattening and saving the layers to do a quick render in Lightwave

The main thing the texture lacks at the moment is 'character' since that's a final stage of the texture-making process, I'll be referring people to your grime-making tutorial to help them with that
