I didnt´said that I cut the exact same frequency on every track... 200 to 500Hz is a huge gap...
Yeah, I was thinking that while I was reading this thread too...
I'm a guitarist, so right off the bat I'll apologize for the fact I'm not as comfortable with other instruments so I'll just use a guitar as an example, but... The lowest fundamental you're likely to get out of an electric guitar is going to be somwehere between 60 (low B) and 80 (low E), so anything below that (and even probably quite a bit around that) is just garbacge that you can pretty much axe without doing anything audible to the guitar tone. Dependinig on a whole slew of other factors, I'll generally cut even higher - say, up to 120-160 or so for a lead track, depending on what it's doing and what's happenign around it, as a starting point.
That means that your lowest band of energy is going to be coming in right around the bottom of this pocket, around 200 or so. That range is going to be critical to how the low end and "beefiness" of your guitar is perceived, and how it'll interact with your bass. Cutting everything through here will pobably neuter your tone; while this may technically represent a "low-mid" range, for me this is more about how the bass frequencies in the guitar will be perceived.
At the same time, I'm not big on low-mids on "heavy" rhythm guitars, so I'll usually at least play around a bit around 400-500 or so, notching a few dB and seeing what happens if I bring the bass up a little here. Sometimes it works, sometimes it doesn't, but it's definitely an area that defines a lot of the "sound" of the guitars, and while I maybe carve a little out, I'd be hesitant to take too much out, for fear of turning my Rectifier into something that sounds like it was a small, solid state Crate. Again, I'm not huge on having an overly low-mid heavy rhythm tone, but an electric guitar is basically all midrange and indiscriminantly cutting is a recipe for an artificial sounding mix.
Then again, half the guys on this board could probably wipe the floor with my ass behind a board, so take this with a grain of salt.