The life of a cell is rated in hours of operation. Therefore, boost cycles will take away from the hours of life. Occasional boost cycles won't affect it in the longrun, but the question for using it on start up is, why?
Most here will recommend adding chlorine manually, rather than taking away from cell life.
If you're just in need bumping up the chlorine level a little, such as for pool parties or bad weather conditions, boost as needed. I wouldn't recommend using Boost in place of superchlorinating your pool or for algae treatments.
Regarding when you can boost it, it doesn't matter if the generator light is on or not. If all conditions for operation is normal, you can boost it anytime. It should activate the cell go into boost right away. If the generator light is not on when you boost it, it should come on.