It's all down to how it sounds best to you really, but for a basic setting, most people use 80hz as a starting point - then just experiment from there.
Some people overlap the settings, some run the front speakers full range - ie with no filtering or "flat" - provided you're not boosting the bass too much, or using the loudness feature, this can work well. Others underlap the settings - ie sub LPF @ 80hz, front speakers HPF @ 100hz.
The settings on the amp aren't a total cut-off point, but a point on a diminishing curve - the steepness of the curve depends upon the "slope" of the crossover. This means that front speakers set with an HPF @ 100hz will still be getting some frequencies lower than this, just at a reduced level, and the lower you go, the more the level is reduced.
