This is my understanding of it, keep in mind that I fly for a small operator and don't utilise these procedures myself so someone else may be better placed to asnwer.
Operators will not assign speeds explicitly, but will tend to specify a Cost Index. This is a value derived from operating (time) cost / fuel cost. The higher the cost index, the higher the speed and vice versa.
With regards to informing ATC, flight plans have to have the planned speed on them, and if the speed you want to do/require has a difference of more then 5% TAS or M0.01 of the flight planned speed, then you must inform ATC, who will grant or deny the speed change unless it's for safety reasons when it is ultimately down to the Captain.