"Pokémon TCG Tournament Rules
6. Shuffling
Each player’s deck is expected to be fully randomized at the start of each game and during the game, as card effects require. In order to achieve randomness, players are to riffle, pile, or otherwise shuffle their decks until they are satisfied that the deck is random. Randomization must be done in the presence of the player’s opponent and must be done in a reasonable amount of time. Care should be taken to ensure that the cards in the deck are not harmed or revealed during the shuffle.
After the shuffle, the deck must be offered to the player’s opponent to be cut once. Cutting the deck consists of creating two separate stacks of cards by removing a portion of the top of the deck, and then placing it under the remaining portion. Players should take care to not reveal any of their opponent’s cards while cutting. Cutting into more than two stacks is considered a shuffle.
Instead of cutting, the opponent may choose to shuffle the deck. This shuffle should be brief, and when it concludes, the deck’s owner is allowed to cut the deck once as described above. Players should take care when shuffling an opponent’s deck, as the cards in that deck are not the shuffling player’s property. At this point, the deck should be sufficiently randomized to both players’ satisfaction.
If either player still does not feel that either deck is sufficiently randomized, or if a player prefers not to offer his or her deck to an opponent for randomization, a judge must be called over to shuffle the deck(s) in question. No player is allowed to shuffle or cut after the judge’s shuffle.
Any action that places cards in a specific order or reveals the position of any specific card within the deck immediately negates any previous randomization. Players engaging in these or other questionable shuffling methods may be subject to the Unsporting Conduct section of the Penalty Guidelines. Players are strongly encouraged to shuffle their opponents’ decks at Premier Events."
In my opinion, as long as de-clumping is performed to remove cards from being in a specific order (i.e., being clumped together), rather than placing them in a specific order, and is followed up with good shuffling designed to randomize the deck, I think it should be legal. Again, the point is not to increase advantage by having cards be in a specific order, but to decrease the disadvantage of having them clumped together. This is followed by good shuffling where randomly the cards could return to being clumped or could more likely remain somewhat spread throughout the deck. De-clumping is a necessary evil because good random shuffling does not seem to do a good enough job of eliminating these clumps.
Now if you see someone grabbing specific cards to keep together in a particular clump in the deck, and then they perform only limited shuffling that obviously does not randomize the deck, a judge should be alerted at once. Grouping cards should not be allowed. Moving single cards to spread them out in the deck, to me, seems like it would be okay.