"Sufficiently Mixed"

Just rule that you should use opaque sleeves in a tournament make it easier for everyone.

Or to answer Ankha's concern :

"If your deck contains different sort of backs for your cards, you must use opaque sleeves"

So you can come to a tournament with a just opened starter deck and still play it without needing to sleeve it.

Ankha wrote: Distinct backs should be equally distributed, with a maximum variation of 10%.
Also, for each distinct back, the type of the front card should also have the same ratio than the whole deck: for instance, you can't have only Jyhad combat cards while the rest of the deck uses a different back. If your deck contains 40% of combat cards, then 40% of the Jyhad cards should be combat cards (+/- 10%).

Since this is quite complicated to achieve, I strongly recommand using opaque sleeves, or not mixing card backs.

Thanks for the answer. Since it's still pretty ambiguous - tournament rules should be rules, not recommendations - I strongly suggest that you update the tournament rules to reflect this ruling.

There has to be a way to solve this problem that doesn't involve: use sleeves. While I sleeve my decks not all players like to do so. I frankly find the idea that we're forcing sleeves on players troublesome.

Can we allow players to keep their library in a tuck box? This way no one knows what the back of the top cards is?

Or we could just stop micromanaging this particular point. There are a thousand other ways to play with marked cards, voluntarily or not. Cards will wear and tear, so will sleeves. Even new, some sleeves have very unequal quality and can be recognized. Since the back of the card is not symmetric, one can even just turn some cards upside down to mark them. The focus on one particular mark while overlooking others does not seem rational to me.