I think 1/2 and 1/3 are quite nice as far as numbers go! Also 2/36 = 0.5555... is not that far from 5%

But (assuming two dice rolled at the same time) 2/36 is something like "roll a five and a six" or "roll either a pair of sixes or a pair of fives". Neither of which is quite as intuitive or commonly used in game design as "using one d20, roll a natural 20"