I wasn't aware of anywhere that doesn't use the full 120sec/180sec as required. You can get away with rounding down en-route separations (sometimes) but never vortex wake when it's a life-or-death issue, let's be honest.
The difference between roll times for small to large aircraft might only be 30 sec or so, but over an hour at a high-intensity airport, that adds up to quite a lot. In the UK it's timed nosewheel rotation to nosewheel rotation, with the takeoff clearance being calculated beforehand to allow for the expected roll time, the aim obviously being to achieve as close to the 2 or 3 min without going a second under!