With a macro lens I usually start at f/7.2 - f/10 and depending on how much I want in focus (like do I want the head AND the whole body super sharp- then I want a larger number, def over f/10). Shooting close up (where you are also getting physically close to your subject you have to make sure the depth of field is not too shallow or you won't get as much in sharp focus as you would like).
http://en.wikipedia.org/wiki/Depth_of_field But you also have to have REALLY good lighting with most standard dslr cameras because after ISO 800 on some of the lower end models it can make the image really grainy. And while for a lot of work it's not a big deal- macro looks best clean free of too much noise/grit. Some mid-levels dslr cameras will still look clean at 1600-3200iso with great exposure. In these cases even though I have an upper range camera- I still try not to let my ISO go over 3200 for macro. 6400 starts looking pretty dirty and can muddy up the crisp sharp detail I was going after with the macro. If the space and subject allows- a tripod or monopod becomes useful. Letting you shoot at lower shutter speeds, with lower iso = a cleaner image.
