Well, i'm not sure what it is you want from an ATCO point of view.
If you are referring to the altitude reporting for radar then the pressure you set in your altimeter has no bearing on the altitude information we receive from your Mode C. This is because mode C always gives us the altitude with the pressure set at 1013mb/29.92in. Our radar systems will then apply local pressure settings to each readout where it is relevant.
So maybe it's more a procedural flying question? From my point of view as an ATCO, if I had given you a decent from FL200 to A5000, I would want to be confident in the knowledge that if I stopped you at A12000 then you would do so with the correct pressure setting.
Having said all that, I seem to remember a pilot telling me that as soon as you hear climb FLXXX or climb to Altitude XXXX then you make the neccesary adjustments to the Altimeter.