Okay, let's me stop being such an asshole. Let's say that I proceed mixing in the knowledge that we all hear 1-5 KHz better than other frequencies. Well, what difference does it make? Will I mix any differently knowing that? Or will I just mix as I usually do, according to how it sounds? (Yes.)
It means that at all velocity levels the ears are more sensitive to certain frequencies than others, in other words loudness is not determined purely by the dBu level out from the converter, it is determined by what frequencies are played how loud at the particular dBu level. Let's say for instance you want your mix to match the loudness, clearity and dynamic level of a commercial mix, then depending on your skills with the fletcher-munson curve, the result will vary.
On one hand you might already have a frequency distribution that makes the signal clip or become very transient harsh very quickly when you approach the target loudness level by adjusting gain/limiting, on the other hand you might already have a frequency distribution that makes the signal not clip or not become very transient harsh very quickly when you approach the target loudness level by adjusting gain/limiting. These two scenarios are a result of whether you are aware of and take advantage of the fletcher-munson curve or not.
If you follow your well trained ears and don't take advantage of the fletcher-munson curve, then what is going to happen is that when you A/B your mix against your most favorite reference track, you are in most cases not going to give your mix better properties - you are going to conclude your mix is slightly less clear, slightly less dynamic, has a little less depth, is a bit harder on the ears and does not respond in the same way at various loudness levels, something is missing but you cannot say what it is. That also means your ears are going to tell you what is wrong and what needs to be corrected - but in this case they don't tell you how, the how means you need to change the core balance of your mix, something you don't immediately understand when you're at the end of the process and think you just need to do a few fixes and it will fly.
What it really is though is transient harshness, lack of depth and too high density, that's the difference. Because when you are not aware of the fletcher-munson curve there is a great chance you slowly push the mix off the cliff when you approach the target loudness level, the distortion is not very audible and you might not instantly notice how dense your mix really is in comparison, but these properties are likely found there.
When this happens you need to do some re-balancing work, based on the fletcher-munson curve and once you get very used to this you are going start dial in bright mixes that end up having the right transient softness, good depth and not a too high density at the target loudness level. This sound is kind of rather dynamic, rather loud, rather soft, rather bright with a good amount of depth.
What you can do to sort of align to the fletcher-munson curve without letting the process become too brain oriented and not ear driven enough, is that in the mix balancing process learn to feel the density and balance towards lightness/low density. This means you can still use your ears as normal, but the density you monitor with your feelings. This results in a bright/light mix going into mastering. Then, even if it is too light in density going into mastering, it will still be easy to make it right in the mastering process (unless you do some really strange high pass filtering on it), much easier than having a balanced but very dense mix going into the mastering process.
When I do the master I use different speakers to instantly tell me exactly how much signal I need in various frequency bands. If I receive a low density mix that is already pretty close to the target loudness level, then it's piece of cake to set the loudness and monitor how much more signal I need in the the different low frequency bands and adjust it to make the density fall into place. On the other hand, if I receive a dense mix far from the target loudness level, it's a more difficult and more time consuming work and the result is more prone to be of lower quality rather than of higher quality.
If you struggle with this and you use cans, and trust me it takes time and a lot of practicing to master this on cans, you can use a set of cans in mixing to set the core balance that help you tailor the mix towards low density in mixing. Such a pair of cans are the Focal Spirit Pro. The frequency curve of these is in a rather straight declining line with too much low end and too little high end, with the cross-over point near 1 kHz.
mix at reasonable levels -that turns out to be just about fine 'Fletcher Munson wise, for a lot of the general tasks, louder at times for other reasons -to hear things differently. But to do that as your norm', is counter productive -for a few real good reasons. And I'd expand that notion in the opposite direction even further; You can also get another completely different and useful view on your mixes at very low levels. Instruments/track balances (but not necessarily tone balances- ), and.. so any tasks you get done mixing at rather quite low levels, is also time you're stressing less your ears.. too.