Should I wear a sunscreen?

Yes, in summer using a sun-screen with 30 SPF is recommended - remember that microdermabrasion removes the outermost skin layer, and this layer acts as a natural sun block for your skin.
Look for sunscreens for sensitive skin with as many natural ingredients as possible.  Physical sun blockers such as titanium dioxide, and zinc oxide rather than the chemical versions have been debated to be safer.
We recommend using sunscreen year round if you wish to prevent pigmentation changes, such as freckles from darkening once faded after treatment with microdermabrasion.