The parallel processing is the key to designing real "fat" sounds, somewhat. The inst.FX are the most easy way to work this out. I really hope renoise 3.1 will allow sending the inst.fx slots to dedicated tracks instead of just outputting all slots to one channel. Given in redux you can choose output channels of the plugin, this makes me eager to see how this will work out in next renoise version.
I also like to split into further ways, i.e. for a reese that'd be sub, sub harmonics with chorus, body wide, processed body mono, upper harmonics - in addition to filter pathways where filtering happens in parallel. It gives real good control over aspects of the sound, and makes some effects possible at all. But take care there's not too much overlapping in frequencies between slots mixed together later, some effects can happen to have side-effects then shifting the sound to something strange (namely filters, eqs and such with different curves). Sometimes this comes cool, though (i.e. what I name body wide and body mono, can make a sound from allto plain stereoised into something very round by adding back a mono signal of the wide original with different charakteristics).
Also, although it needs more fx slots, I found it useful to make "spread" (where sends are chained pointing to the processing slots) and "recombine" (where sends at the end of those slots send to on mute mode) slots for parallel stuff, to get better control of the processing chains, also more control over levels/dynamics and mixing components. It can also help keeping view and track of complicated chains, though you need more slots to work with.