What should you do? Should you cut hair when it is wet or when it is dry? That depends.
If you are cutting hair with scissors, you should be cutting damp hair. If the hair is damp, it can be combed neatly into place and held securely in your holding hand with all the hairs lined up perfectly for the scissors. If the hair is dripping wet, you'll have a soggy mess on your hands with hair clumping and water dripping all over the place. The key word here is damp.
If you are cutting with clippers, the hair needs to be dry. Wet hair clumps and therefore does not feed evenly into the clipper. This will give you an uneven cut. Wet hair bends away from the blades.
However, if you are about to cut someone's hair with a clipper and he has a classic case of "hat hair", you'll first need to wash the hair. Wash the hair and dry it before clipper cutting. You want the hair to feed evenly into the clipper and kinks will bend away from the clipper and give you a very unique looking haircut!