Although I personally believe in the eq'ing of a track after compression I do believe that Hp and Lp eq must be done first.
I do roll off the unwanted lows and noise before I start the process of mixing. This to me is very important especially if you believe in compression first. I think a lot of people that say they compress first forget to let people know that the unwanted lows and highs must be eliminated.
However, saying that there are some who do compress first without rolling off the lows and highs. So its kind of a what works for you process.
If the end product sounds great then which ever way you decide to do it is totally up to you.
There is a lot of unwanted signal at those frequencies below like 40hz and above like 10k. Getting rid of the unwanted noise on all of your tracks will open up room to place instruments in your 3d sound field. Giving you more room to work with in the frequencies that the instruments sit in a mix.
This is a very brief explanation of hp and lp filtering but it's probably the way it is used most often.
I will be posting about the most popular eq's used and their use soon. So please come back and visit my blog again.
Thanks
David W. Aucoin
David Wayne Recording.com