My best answer is.... that it depends!
The biggest factor, I think, is the amount of error in your data. If you have pretty good data you may be able to get away with smaller values. Lots of noise? Gotta crank it up to get what you want.
I generally find I need to use values a little higher than what the defaults are in the software.
Also, sometimes I prefer a more simple network, so I crank up the 'contraction' slider all the way, regardless of how accurate I think my GPS data is.