At every powering on the machine should be homed. Period.
Suppose you have micro stepping drives and when your machine is stopped at the end of your day, motors are at "some" micro step position.
When you switch power off, motors will spring to the nearest FULL step position, defined by the magnets in the motor.
As drivers are not intelligent enough (read: dumb), they will at power on be at the full step position, and the result is, you may guess :) : unreferrenced machine.

Lazynes for clicking at one button after power up?