Unless you have been asked to "Report leaving FLXXX" then there is no need to make the report. It is helpful for Controllers when you do it though as it brings you back into the forefront of our thinking, e.g. To QSY you shortly or to give you further descent. If you were given a "When ready descend FLXXX" quite a while back then you could have moved down the priority list in our mind (as we know that you are sorted clearance-wise etc) so by giving the report then you refresh things for us.