It isn't cheating, because if it were then when you put your options for a card like Twins near the bottom of the deck in order to have all of your options next to each other, then that would be cheating as well. Furthermore 'declumping' cards, is used to decrease the odds of cards from sticking together, and yes it might be a result of poor shuffling, but I hardly see why that matters in context to anything enough to give people warnings, game loses, or even DQs, for. It's a harmless habit that might influence the game, but it will influence it in the same way as putting a few cards next to each other so you can organize your thoughts. If you aren't satisfied with the way they shuffle then shuffle your opponent's deck, the solution is that simple. People don't declump to cheat, people declump, because they believe it might stop dead draws, and it isn't something that significantly alters the game.