Interesting point raised by Chilli that is missed (from an understanding perspective) by most (if not all) pilots.
If ATC ask you to be level, for example FL270 BY RESNO, what they really want is you to be level 5 miles before that point.
That's about 40 seconds before you get there.
What this does for the controller is give the assurance that you will actually be level at the point and remove the need for any coordination (because 9 times out of 10 the LEVEL BY requirement is on a sector boundary).
What happens too often is a VNAV descent that the computer on board initiates as late as possible before the waypoint. This give ATC the heebyjeebys because it looks like you've forgotten to descend or will be too high.
In short - go down early. Wont kill you, since you dont really care what level you cross the point at do you?