if speed control is applied, ATC expects pilots to stick to the instructed speed +/-5%. a frequency change doesn´t cancel the speed instruction.
of course ATC sometimes is to busy/distracted to cancel speed instruction, at this time it would be the best if the pilot requests or states the wish/need for speed reduction.
nothing worse than having an approach sequence and one pilot reducing speed without notification. this usually causes the immediately succeeding traffic to close up too soon and might end in missed approaches or aircraft being taken out of the approach sequence (most of the times the aircraft being taken out of sequence is the succeeding one and not the one that reduced without notification, because maintaining separation behind the one aircraft that reduced speed is harder to do as the distance gets unexpectedly less).