As has been said earlier, slow as quickly as you comfortably can.
If you are under positive speed control then deviating from the assigned speed without requesting it is really a no no. If you have an operational reason (i.e. turbulence) then state it and no controller should refuse this request.
A real bugbear of mine is those who slow from an assigned speed without a word. 180kts 20 miles from touchdown is unreasonable in most circumstances when there is a lot of traffic about. I'm sure you'd get a bit miffed if we slowed you to 180 with 20 to go every time wouldn't you?
I have never had any problem with Ryanair. They invariably fly what is asked and will often fly faster for longer than other similar types if requested/permitted. This permits more flexibility on the part of the controller and often allow a sequence to be run more effectively than could have been the case with everybody slowed up.