I personally see no reason to make it a requirement that people must vote on X amount of categories so that their ballot is valid. Some people may only have interest in one category, and so it should be their prerogative to be able to vote within it without having to put forward votes they don't have any desire to. Naturally it would be best if people voted in as many categories as possible, but that's an idealist positive which I do not think would be subverted by allowing people to vote for just one category at minimum.