I try and keep all my cuts and boosts under 3 decibals. Cuts are not as important to keep that rule and boosts in my opinions.
Keep this in mind. I find that if i ever make an EQ adjustment somewhere that is that high in decibals, its probably more to do with the fact that im doing it in the wrong place. You might not be hitting the root of the harshness. But the problem is you need to have good monitors to help you find the real root.
If you know your hitting the right spot and doing more then just do it. It probably wont sound bad if it really is in the right spot because then you are correcting it. If you are doing it in the wrong spot your cutting decibals to areas that arnt a problem around the problem area, which will cause more problems and probably make the real problem worse. Get what im saying? Your cutting out things around it and it brings that problem out.
With the right tools, right monitors, and the right ears it doesnt matter as much when it comes to mics if its generally good and balanced. But for someone who doesnt know what they are doing they generally do more harm than good when they use EQ. Thats why it really is just best to do it right the first time.
Danny