Sisiutil
All Leader Challenger
The main reason I avoid automating workers until very, very late in the game is a point alluded to by smackthewise: the AI doesn't really understand city specialization, whereas a human player does (or should). The improvements around a city need to be chosen to support its specialization, and you can only do that by manually controlling your workers.
Once the late game rolls around, however, I often start automating them, because there can be a lot of other things to micro-manage in the late game (especially if you're at war). I only do this after I have Railroad and I've built RR on all the tiles where that provides +1
and I've connected each city to every other city by a direct rail route (taking advantage of the aforementioned production tiles where possible). And like smack, I consider "Leave forests" and "Don't build over improvements" to be essential settings.
I should also mention that I will not automate workers if I am fighting (or anticipating fighting) a war on my continent. I may not miss a few workers captured near my borders in the late game, but it will add to my war weariness, and we all know what a bear that can be in the late game.
Once the late game rolls around, however, I often start automating them, because there can be a lot of other things to micro-manage in the late game (especially if you're at war). I only do this after I have Railroad and I've built RR on all the tiles where that provides +1

I should also mention that I will not automate workers if I am fighting (or anticipating fighting) a war on my continent. I may not miss a few workers captured near my borders in the late game, but it will add to my war weariness, and we all know what a bear that can be in the late game.