Doc 4444 says:
8.5.5.2.2 Aircraft maintaining a level. An aircraft is considered to be maintaining its assigned level as long as the
pressure-altitude-derived level information indicates that it is within the appropriate tolerances of the assigned level, as
specified in 8.5.5.2.1.
8.5.5.2.3 Aircraft vacating a level. An aircraft cleared to leave a level is considered to have commenced its
manoeuvre and vacated the previously occupied level when the pressure-altitude-derived level information indicates a
change of more than 90 m (300 ft) in the anticipated direction from its previously assigned level.
8.5.5.2.5 Aircraft reaching a level. An aircraft is considered to have reached the level to which it has been cleared
when the elapsed time of three display updates, three sensor updates or 15 seconds, whichever is the greater, has passed
since the pressure-altitude-derived level information has indicated that it is within the appropriate tolerances of the
assigned level, as specified in 8.5.5.2.1.
One might think that it is unnecessary to report leaving and reaching but still it is quite a common practice. I just can't find any documents supporting this practice.