When I'm building up my base, I sometimes find myself wasting time trying to get a single worker off of my resource line so that I can build something. I often miss or have to chase after them if I'm trying to single click, and dragging often selects more than one, at which point I have to try again or click on one in the selected units list. It seems like it would be best to grab one right after it's returned its resource and is heading back for another, but they're also moving the fastest at this point. It's easier to grab them while they're mining, but then they're carrying resources around and this feels wasteful. Is this minor optimization unnecessary, or am I missing something? Should I just keep a dedicated builder idle for easy selection (as terran or protoss)? Put one on a hotkey?
So obviously selecting 1 is better than a group, but a little known fact:
Workers smart cast!
This means if you select a group of workers and try to build something, the nearest worker (and only him) will go to build the building. Additionally if you build more than one thing, the two nearest will go to do the task. This means you don't need to worry about selecting more than one (provided you don't issue any move orders), just select the group and build away!
Don't select all your workers and use smart cast, make sure you only grab workers that are on minerals. You don't want to pull a worker from gas and then later put him back on minerals. You'll be short a worker on gas, which is very easy to miss.
In the early game, when there isn't that much to do, you should get used to selecting a worker individually on his way back from mining, then watch him drop his resources off, and then tell him to build. This is better than smart casting. Later on when you've got a lot to do this'll be way too time intensive, so at that point you can switch to using smart casting.
Also, tell your worker to build the building, and then shift right click on your minerals. That way he'll go back to mining the moment he's done, which means no time is wasted.
No, don't have any builder idle if you can help it. Really I just single click, I know you tried this but I would guess that is the best way to get just one. Just spam click if you have to until you get one, eventually you'll get better at aiming anyways.
Another note: If you hit F1 it will select a worker who is doing nothing!
One note to make if you select 5 workers and hit B(build) S(supply depot) and click where to build only 1 scv will go to build it even though you selected 5. Obviously this still works no matter what you build but you don't have to just select one you can select any number of them and just give one command. Each command you give after that the next one will go and do the building.