Yes 2 to 2 1/2 stops down from wide open is a good general guideline, most lenses will be at their best around there but not yet have reached a point where diffraction begins to have an effect. It depends on the lens of course, a very well corrected lens may peak only one stop down, a fast lens with coma and astigmatism may need to be stopped down 3 or more stops before the aberrations are minimised. Often the centre improves before the corners do, so it can be a question of degrees, and as David mentioned, the performance of a zoom can vary with the focal length.
The sweet spot is really a balancing act between the aberrations present when a lens is wide open, and the gradual loss of sharpness caused by diffraction as the aperture gets smaller. For 16mm format, where a circle of confusion figure of 0.015mm might be desired, stopping down past f/11 will create an Airy Disk (the blur circle of a point source) too large to contain within that CoC, so for critical applications f/11 is about where the effect of diffraction begins to become noticeable.
We have an Angenieux 11.5-138 in our rental fleet. At the wide end it conforms to the 2 stops down guideline, at around a T4/5.6 split the aberrations have mostly receded. Towards the long end it probably needs 3 stops, so maybe T5.6/8.