The auto-bed leveling on your 3D printer is nice, but it’s not a replacement for manually leveling the bed. Manually level the bed before every print, since taking off your last print from the bed has the tendency to make your bed not level. Then, after manual leveling, do an auto-bed leveling to remove the remaining tiny variations in the bed level
Depends on your printer, I think. Mine is quite happy being leveled only after nozzle changes, and the autolevel is Good Enough.