YOu should be scooping out the muddy frequencies on the offending tracks while mixing. EQing the suspect frequency range during mastering is just going to lower the volume of the mud in relation to the other frequencies.
There's really no rules.. every song/project is different.
If you're mastering your own mixes there shouldn't be to much eq needed because you're using the same room that influenced your previous eq decisions in the mix .. so that shouldn't change drastically. If anything I like using wider bells and rarely use notches .. a dB or 2 where needed.
There are not really any eq rules that pertain to mastering as such. Ideally a mastering engineer would need to eq as little as possible, this requires a very good mix. Of course a mastering studio should have a monitoring system which is better than that of the mix engineer in order to have authority over what will be an improvement for overall translation. The rule is (if there is one) any eq should have a larger improvement than detriment.
As with just about any eqing situation...
Listen to what you have.
Imagine what you want it to sound like.
Set the eq how ever you have to in order to turn what you have into what you want it to be.
the only sensible rule is USE your EARS!
listen to audio when cutting/boosting,
maybe try M/S - and EQ offended channel (S?)
but if it's your own mix, then well - get back to mix and sort it out on the track -
simply as that
Because I master where I mix (because I'm a cheapskate), there's no point. If I do detect an EQ problem in a track I'm mastering, I go back and fix it in the mix.