The light will be blinding to yourself and more importantly others. If your pubic brush is really thick the tan rays can't get through. Hair is a great blocker of sunlight. That's why when you shave people's heads it looks pale as hell underneath, just imagine that except on your hoagie and meatballs or red bean taco, what have ye. If you tan too much with thick pubic foliage when you finally do shave it off your crotch looks like that transparent headed fish that lives in the deepest region of the oceans. That's not what you want.
If you're tanning on the beach however, that is a situation where you're not going to want to shave. If you're lucky and your downstairs hair is thick enough to cover your entire crotchal region then no one should take any real offense to you being naked on a non-nude beach. This will help you get a complete tan without having to worry about the embarrassment of wearing a stupid looking Speedo. It will also eliminate the need to put sunscreen on your genitals which when taken out of context looks like illegal activity and you will most likely be arrested the same way I was.