There are NO frequencies that are always good to scoop or boost or whatever. The best thing to do is to record your amp (or modeller or whatever you use), then compare it with a tone you like and play with the EQ. It is better to use a little EQ as possible on gtrs but this will help you determine what frequencies are good/bad. Use charts that say 'this range of frequency = honk, etc.' to begin with but you'll learn quickly. I've been doing this on and off for about a year and I can tell, just by listening, where 500Hz on a guitar is compared to 300Hz or compared to 1kHz, so I know immediately what range I'm looking in to boost/scoop. I'm sure the more experienced people will be able to tell with even greater detail. But once you start to learn this, you can compare to reference tones, and listen in different environments and be able to edit your guitar tone much easier.