5MB, if a pilot checks in with me telling me he/she has been given descent with a reduce when level I will either. A) let him level and reduce speed, B) tell him/her to reduce speed and not worry about the height (when appropriate) C) if I give further descent I will specify when I want the speed reduction. I base my decision on what I need and why the previous sector gave the restriction. It's the same with level by restrictions, as if I give further descent it cancels the previous instruction - if I (or the transferring sector) need the level restriction I will restate it.
I would probably be accused of over controlling at times but as others have said we work to the lowest common denominator and if it ensures a safe orderly and expeditious flow of air traffic in the system then that's what I'll do. As much as possible I try to let the pilots do the flying - after all that's what they do!
Cheers
Northerner