I have had it multiple times that my cube was extremely greasy/fatty/sweaty on the exterior after scrambling. I'm 100% sure this is because some people just have sweaty hands, I'd like to ask everyone who has sweaty or dirty hands to either go judge and someone else to scramble, or to wash your hands before scrambling.
At German Nationals I judged someones megaminx solve and the competitor asked my to wipe of the cube with a cloth first, when I did that I immediately understood why the competitor requested this... it was so slippery!
Hygienically though, I think it is a bit paranoid not wanting to touch other peoples' cubes (exceptions are there though, for certain people.. I don't like to touch their cube) 99% of the cubes are fine anyway.