Agree with SM4 Pirate.
Many do it early (e.g. with STAR clearance) because you have to state a level anyway, and if there is nothing in the way then it cuts out a transmission later on. Personally I try to leave it until about 5 minutes prior to TOPD (on estimated profile) because by that stage the pilots a preparing for descent and they are less likely to forget the clearance.
Any info is good info; just as pilots want to know if they are likely to be restricted on descent or when they can expect it, variations to descent profiles are also invaluable. Had several jets that have descended 200 NM Brisbane to absorb a delay in a shallow descent (read: glide!) rather than hold. So it definitely helps traffic planning.
ATCs are also aware that profiles will change, for example, when aircraft are given sequencing instructions that necessitate a go-fast or slow-down, and facilitate accordingly where possible.