A good question. Certainly in the UK a restriction of FL200 20nm before xxx means that you make your descent (or climb if appropriate) to FL200 at any rate you like. We really don't mind how you get there, if you get there early or make a dirty dive at the last moment, but, when it comes to 20nm before xxx we expect you to be level at FL200. There are normally very sensible reasons to actually achieve this level - the FL210 crossing track 5nm after xxx for example - so it is important to let us know if any target level is not achieveable, so we can warm up plan B.
(I'm trying to think of a way to pass an instruction to descend to FL200 20nm before xxx - but not to be level until at least 25nm before xxx - and it all gets horribly complicated and open to confusion and, why would I want to do this anyway? - which is probably why we don't do it)
In short, achieveing the target level early is OK - late, definitely not.