Basically, if you cut, you’re just reducing the volume in the cut area which has no effect other than the obvious reduction of those frequencies. However, your also taking the noise level down which is not a bad thing.
On the other hand, let’s say you have a drum track where there is a very weak 100-200Hz sound. If you boost it 5-10db then you’re pushing a lot of noise to make the track have volume in that zone. There are also artifacts inherent to the plug in, or even hardware, i.e. the tonal characteristic of digital summing or just the sound of the hardware, which comes along with pushing information that is not present in the original signal. It turns out that phasing can occur as a result which makes the boosted area indistinct and not exactly clear. This is why you always hear the term, "Get it right at the tracking stage." When this is done then you have less EQing involved and the tracks sound clearer and more distinct.
Guys who have mixed a lot, and know their gear, are pre-thinking , at least the rhythm tracks, and pick the right mics, preamps, compressors and EQ going into the machine. If you know your room this is invaluable too.
Most of this I learned from guys around here, magazines and the internet so if somebody sees something a little off please feel free to comment. This said, occasionally I will boost but only very small amounts with wide Q.