I don't know how many ATC manuals still have it written, but it used to always be a requirement to report when reaching or leaving an assigned level. In today's crowded skies, it seems a little archaic, but in procedural airspace, it's a nice call to keep you in the picture.
IMHO, it's always a good, last minute check to see if, as an ATCO, you actually have a clear bit of airspace ahead of the subject aircraft as he starts his/her descent. You may have had nothing in sight when you issued the descent, but things may have changed by the time it actually starts going down. Many will say that you shouldn't have issued the descent in the first place if you didn't have a clear patch, but that kind of thinking belongs to those of us who have never issued a dodgy clearance in their careers.
Bon chance!