The best thing to do is to get the sound that you want without having to change it constantly in post production. If treble sounds harsh in the mix then instead of eq-ing it just lower the treble knob on your amp until it doesn’t sound harsh anymore. The goal is to get the sound that you want and fix it as least as possible in post production.
Every guitar sound is different so there aren’t universal frequencies that need to be cut or boosted but I can tell you some frequency ranges and what happens there so you know where to look for what. You can ALWAYS low cut any instrument or sound because you want to have a clear low end and a lot of instruments have mud in the low end that keeps clashing with the bass frequencies and you don’t really want or need that. You can easily cut anything from 100hZ and below on guitars because that’s where the mud happens. You can boost around 150hZ to get that woofy sound. Between 400 and 1kHz depends on the sound, you can either cut or boost in that range. 500hZ generally has a lot of “air” so you can cut that because in that range it just sounds like the wind is blowing. And around 3-5kHz you can boost to make the guitars jump out in the mix, so they are more present. And lastly you can High-cut around 10k, sometimes less somtimes even more, again it just depends on the sound and how much information is in that range.
These ranges are just starting points. It doesn’t mean that this will work for your sound but you will know where to look and if your guitar really jumps out if you boost in around 2k and not between 3-5k, then boost it at 2kHz. These ranges are not written in stone, they are just starting points. You said that your guitar sounds muddy. If you want to figure out what exactly you don’t like about your sound, try using a bell EQ curve and make it cut and then move the curve around the whole frequency range and once you stop hearing what’s been bothering you’ve found your problem.
