Pickups that are properly adjusted for string gauge and balance will not need much adjustment for periods of time. Usually when a player is playing hard the pickups could vibrate and the height could be affected. If other repairs or modifications are done the repairman could change the settings that would make you instrument sound different.

String gauge would be the biggest difference and change the output of the pickup. The string diameter and amount of energy created by it will also change the control setting on your amplifier. Different alloy strings will also change the tone of the instrument. You may need to balance the pickups from one end to the other depending on how you play either using a pick or fingers. Heavier strings will give the instrument a fatter fuller tone especially on the bottom strings and usually by lowering the pickup will help balance the output when strumming.