At a major European airport that I go to frequently, they often clear you to line up in sequence and if the preceding aircraft is a heavy, once it's airborne they say something like, "take all the time you need for the wake turbulence, cleared for take off". This puts the onus on us to decide when we will start the take off roll but as it is common practice for us to start a stopwatch when the preceding aircraft is a heavy, it seems a reasonable practice to me. After all, we are trusted to do plenty of other things ourselves.