I would suggest reading posted reviews of your lenses (and those you are considering), most of which give enough information to deduce where the "sweet spot" is. My first recommendation would the reviews posted by Bjørn Rørslett.
"sweet spot" is a vague term to define an aperture-focal length combination that is apparently well-suited to a given type of shot for a specific lens.
It is not exact, nor do most users have any real clue about where their lens produces the "best" results. I think many users have a good general idea what apertures work well for their needs, but that's about it.
For the vast majority of lenses, the so-called "sweet spot" is the range from f/4 to f/11. With fast lenses, this range may widen up to f/2.8 or even f/2 in some cases. Most will be at their peak performance in the range of f/4 to f/11 though.
I wouldn't worry too much. Precision is not really a factor when people talk about this stuff--nor is factual accuracy necessarily.