I'll add my two cents here... If you have the camera, the lens and the tripod, and are going to be doing some serious macro shooting on an ongoing basis, I would strongly recommend a focusing rail. Even a simple single axis rail gives you so much flexibility in choosing your exact focal point... and they are dirt-cheap on the 'bay. (no help at all with flowers on a windy day, but then again, nothing really helps then... )

Cheers,