I agree with everyone about the importance of calibrating and profiling the monitor.
Expanding Colin's point about "compensate without realising what's happening": what can happen is that the monitor colour isn't quite right, and you adjust the image to look right on the (incorrect) monitor. But what you're doing is to add an error in the image to compensate for the erorr in the monitor. And without a calibrated/profiled monitor, there's no way of knowing that this is happening.