Addendum

[Paeanblack]

This is quite true in almost all cases beyond 1750AD. For the complete micromanager, you should switch from 10% science to one scientist when you have 3 or more cities producing 7 or more commerce or your capital(oldest city) producing 5 or more commerce and two other cities producing 7 or more commerce. Before one reaches this stage, there is no commercial benefit to using a scientist in lieu of working a food-producing square with a 10% global science rate.

Ideally, create one scientist in your most worthless city if you are attempting the 0%+1 science rate to exploit the 32 turn maximum.