I have found from much experience is to keep an eye on tracks at track level for lower mid boxiness and just keep it under control there but don't do anything too radical either. Aim for a well balanced mix prior to mastering. Very slight amounts or a little dip around 300 Hz at track level can work but not at the expense of the sound either. Use your judgement there.
Mastering
is a good place to control this lower mid problem further.
(not at the time of mixing and using EQ on the masterbuss, a totally separate mastering session later) The reason is that a mix tends to build up a little excess energy there for various reasons. A good mix will only need a little dip between 200 and 300 Hz, not very deep (-3 to -4dB max) and a gentle bell shape as well.
(bandwidth should not be over wide either from 150 to 350 Hz max) The trick is to use this little dip in a minimal fashion and a mix can just become clearer all round when the right amount of this is applied. A good mix may not need a lot or any of it all either. Use your ears again to judge.
If you have been mixing all day you might become a bit immune to any build up of energy in this critical area. So it is best not to apply any eq in this area in your masterbuss. Another reason for mastering a week later. You will hear it fresh and get a true indication as to how much build up there is there.
I have been giving it some thought. Middle C is actually only 261 Hz and this is almost considered the centre of musical pitches. Bass is one and two octaves below that and the higher notes are one and two octaves too above that as well. Fundamentals don't go up much higher than 2KHz or more so most of the energy above 3Khz - 20Khz can be considered to be harmonics related. When I was studying jazz piano at one point they were telling is to keep all your voicings around middle C because they sound best there. Higher and the sound gets thin and lower it gets muddy.
A lot of the musicians in the music are in fact hovering around middle C. Guitar is from 80 Hz to 600Hz or so, there can be a lot of energy around middle C. Bass fundamentally may not be there but harmonics wise it can be. Keyboard players are playing a lot of voicings there. Some of the lower horns will sound there. It may explain why there can be a build up around this area.
In mastering you can just put that nice little dip in there at the right depth and it all seems to just even out nicely. Trick is as soon as you start dipping too far down you are now pulling out energy there. Practice listening to very small amounts of EQ changes and revel in how massive the change really is. People are not tuned to it and are making way too drastic eq changes and wondering they have just smashed their mix or their track. Listen to how amazing 0.5 dB of boost around 1Khz sounds effecting from 500 to about 3 KHz only, very gentle bell up just a tiny tiny amount. Wow listen to how far forward the mids just moved.
In mastering dipping down 2 dB at 250Hz may not be enough and -3db too much so you set it at -2.5 dB instead and it might just come out perfect. Good thing to do about now is switch to your reference tracks at exactly the same volume and listen to how clear things are down in that area too. Go back to your mastering and fine tune etc..