Re: Guitar frequency range...
The high end of the frequency range comes from overtones and harmonics, and their prominence will depend on you pickups, pickup placement, wood, amp, cables, pedals, speakers, picking style, picks themselves - the list goes on.
As to which frequencies should be cut, there's no answer to that other than to listen through some decent monitors, and adjust the EQ accordingly to your liking.
If you're in your teens, nothing over 20khz matters. By your late 20's, your ears cutoff will probably be around 18khz. In your 30's it will be 16-17khz, generally dropping at least to the 14khz range by your 40's, and down further as you go on. Of course this degradation goes a lot faster if you don't take care of your ears, and always wear hearing protection in high volume environments.