I started Use Small Aperture and it was close before I could come back to say something. What a heated debate!
Some gurus said that optimum sharpness is obtain when you stop down by 2 stops or so, as a general rule of thumb.
When you shoot close-ups like flowers, you want optimum sharpness in every part of the flower, so that the optimum aperture could be somewhere at F16 iIMHO.
If you do not print larger than 8R or A4 size, Optimum aperture is preferred to optimum sharpness. Just my 2 cents worth.
and I heard someone saying extra small aperture of F64 - another guru was saying that at smaller aperture than F22, there is a beast called diffraction to attack your pix.