When you can save worker turns by not combining workers, most likely in the early game, it makes no sense to combine workers. A turn saved at t30 is 10 turns saved at turn 100. It is very important to micro them as much as you can to reap the full benefit early on, because the early micro will snowball later on. After you have built all the early improvements and the only improvement you have left to build is roads it makes sense to group your workers, or even automate them to expand your trade network.
I usually tend to stand grouping my workers around 1AD, after all the most important chopping etc is done.