If you are talking about hand adjusted EQs, they should be used very sparingly. As Jake notes, many users in the past cranked the bass and the treble and thought things were better. The problem with that approach is that a lot of the sound we hear is in the midrange. By changing the relationship between the bass, midrange and treble, we skew the entire presentation.
Lots of boosters of the Auto Eq features (on receivers) that are out or coming out swear by them. They are pleased with the changes the auto circuitry makes to the presentation. From what I've read, most of these EQ's seem to make things clearer, so that translates to the mids and the highs being boosted.
Also, another issue is that some of these changes often involve very large boosts (as much as +6db) at some frequencies which can tax speakers and the receiver's amplifiers. IMO, one should be very careful when equalizing upper frequencies (IOW, above the bass.) The wavelengths of the sound are so short that if the response is not averaged over the entire listening area, the sound may be fine in one listening position and worse than where you started everywhere else. None of the currently available receivers out can average upper frequency response for more than one listening location.
The best approach for good upper frequency balance in a listening room is buying the right speakers to begin with, along with placing them reasonably well. Of course, the room itself is a factor too. If it has too much of an echoey sound to it, it can effect clarity.
Also as Jake notes, bass EQ is THE type of frequency response control that an EQ should be used for. Bass response peaks or nulls are something that room placement can only affect a little. And, these peaks or nulls are spread over a much wider area than the upper frequency issues. (This is because bass frequency wavelengths are so long, the opposite of the high frequency problem.) Set a sub here, you have one problem. Move it a few feet, you have another. This is where a bass parametric EQ can make a real difference. To date, none of the current receivers (although the new Yamaha flagship may be able to do some type of bass EQ) are capable of doing room/bass correction correctly (parametric style EQ with no set frequency bands.)
Bottom line, my opinion is that in general, most EQ's are not a panacea for HTs. For bass frequencies, if they are corected properly, they can be quite helpful. Move higher in frequency and the benefits are not as clear cut.
(Here is a link I've listed before to a good paper on EQ. While it does talk about a product, the info on EQ is enlightening.
http://www.meridian-audio.com/w_pape...ection_scr.pdf )
(FWIW, the above is my opinion. I know that many users of the auto EQ units will disagree. And they are certainly entitled to their opinion too.
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