Well to be honest, the only books I've really had to do any editing on were plain text files, just made sense to clean them up in the word processor and import them into calibre for conversion.
All the epubs I've downloaded have been well formatted so I haven't really had much of a reason to use Sigil.
Of course, my formatting standards are pretty low

I find I can comfortably ignore minor errors, so I'm usually quite happy with the output I get from calibre.