Dream tan is fine, but the benefits of a tanning in the sun is much better phyisologicaly, besides helping you get a great tan (provided your body type is capable of that) the sun is a dramatic aid in ridding water and making your skin a much tighter and giving a drier look.
Bodybuilders today have become so reliant on drugs and fake products, I seriously don't think they take into consideration of using natural and old school methods. If you time your tanning properly before a show and regulate your water intake, there should be no need for diuretics. Todays contest prep has becaome as lazy as their training methods!
BTW, if there is no sun.....use a tan bed, ALMOST the same effect!!