As spray tan technicians who have been in the industry for a while, we always tell clients to exfoliate. It's a must. Why must you? Because we have dry/dead skin that covers our bodies and THAT skin will absorb more tanning solution than healthy skin. Dry, dead skin is yucky. In the spray tanning world, it's called "alligator skin" and we can minimize the problem by sloughing off the old skin with scrubs or loofas.
That being said, you CAN exfoliate too much and cause irritation to your skin. Your skin will then OVER produce oil to compensate and you're stuck with oily skin that won't properly absorb the tan either.
Our pro advice? Exfoliate your skin every other day - with a gentle scrub. Don't feel the urgency to scrape your skin dry the day before your spray tan - if you're doing proper exfoliation every 2 days, you will be perfectly primed for your tan. We recommend our Body Scrub (but of course...) because it's made with all natural ingredients specifically designed to exfoliate your skin while keeping it hydrated.