Lol I know a little bit about this cause I played sp for bout a year so here's my advice.
It all depends on the economy you have at hand and your intentions with the resources behind it. If your building a granary then its more than worth it to get new citizens cause the faster you build economic buildings, the more resources you obtain allowing for this dillema not to appear in the first place xD.
It also depends on your building and its other purposes. If you expect an attack a castle can mean the difference between keeping or losing your city and all its economic contributions, so in that case spawn new citizens AND use active ones if neccisary! But REMEMBER! Even 20 seconds of buliding from active citizens could stagger your economy so only use active citizens if time is of the Essence. You lose more resources from taking workers away then simply adding more while the resources replenish.
As for keeping an extra citizen for building, I say it depends on your intentions. Putting groups of idle citizens free to build in cities could be optimum in overall infrastracture if your planning to build much more. However note this is only efficient if you plan some economic boom with the buildings you make or if your going to build very soon. Otherwise than yes your totally wasting your resources by depriving your workforce.
To me the scouting is an ez choice. Just bring them back when their done scouting its only 2 citizens that can be replaced in a heartbeat.
I hope this helps and see you in the servers!
[This message has been edited by Desert_Fox_42 (edited 04-16-2013 @ 11:19 PM).]