Is Mana Weaving ok if it's followed by a thorough shuffle?
Look at it this way:


If you properly randomize your deck, then its initial configuration is irrelevant. So "mana weaving" is just wasting more productive time.
If you don't properly randomize your deck, then "mana weaving" is likely part of you cheating.


So, there's no upside. "Mana weaving" doesn't actually accomplish anything, unless you're accidentally or purposefully cheating.

Judges routinely prohibit "mana weaving" because the down side of disrupting someone's pointless pre-game ritual is much smaller than the upside of guaranteeing a fairer game.



Some players "mana weave" to "break up clumps." That's based on a mistaken assumption. A truly randomized deck isn't one where the lands and spells are perfectly distributed; it's one where all the cards are in an arbitrary and unpredictable order. That'll mean you sometimes get lucky and sometimes you won't. Bad draws can be frustrating, but some amount of clumping &mdash; even the rare incident of really egregious clumping &mdash; is perfectly normal. A player who never gets mana-flooded or mana-screwed isn't shuffling properly.

If you're constantly experiencing clumps, especially drawing cards in an order similar to how you played them last game, then it's a sign you're direly under-shuffling. Spend more time shuffling (2-3 minutes before a game is pretty standard) and practice stronger techniques like riffle or mash shuffling.