Audited the code - in gallery3 and gallery3-contrib, we only ever use this to set the rand_key and then use it for random queries.  With that context in mind, I changed Random::percent() and returned Model_Item::__construct() back to the way it was:
https://github.com/gallery/gallery3/commit/9c3ac112e3e1f4939916f6f039f6e1dbbe14aaad

I chose 10 as the default number of digits because, well, that's how we defined the items table :-)