It is important to get the correct saddle height, otherwise you will be more prone to injury. Also if the saddle height is wrong your cycling will be less efficient.
When making adjustments to the saddle position it is best to make small adjustments at a time.
When you have found the correct saddle height and you are happy with it, make sure you keep this same saddle height for all your bikes. This is especially important if you do a lot of cycling and have different bikes for racing and training.
One difficulty I have when measuring the saddle height is – what actually is the top of the saddle? It depends where you take the measurement on the saddle. – e.g. saddle is unlikely to be perfectly flat I try to take the measurement of the saddle in line with the down tube and the heighest part of the saddle (i.e. in the centre of the saddle).