a tight nock will give an inconsistant release off the center serving as some of the energy from the limbs, via the string, will be lost as the nock tries to free itself from the serving.
for a split finger shooter (3 finger mediterranean or 2 finger flemish), the nock can and should be loose - that's a benefit of those styles of string grip. unless you have a need for the arrow to not fall off the string of its own accord.
all other forms of string grip need to have the nock grip the center serving loosely, so that a suspended arrow will hang on the string and a tap of a finger to the string drops off the arrow.