Just curious, but if ATC asked you to fly a speed of say 220kts, would you bug 210kts, incase the aircraft went too fast, or 230 kts incase it flew too slow, or just bug 220kts and accept that deviations are going to occur as we fly in a dynamic atmosphere but as long as the speed returns to target within a reasonable period of time and you don't contineously bust it then you've got the big picture covered and won't mess up ATC's spacing?
On a seperate point are there many places other than the USA that really strictly enforce the 250/FL100 rule? In Europe it's almost a daily occurrence to be asked to keep 280/300/330kts as long as you can for spacing with traffic behind/sequencing depending on how busy the airport is and where you are in the queue. Europe seems to treat 250/FL100 as more of a guideline than a speed limit.