(Shuffle Before Game, No Big Deal)
The during the game "Opponent" Shuffle should be frowned upon by judges for 3 reasons.
1. No Reason for it if the deck was reasonably and properly shuffled, Just cut to randominize the top card.
2. Stalling aspect is often there.
3. SNEAK A PEAK INTO THE DECK, by use of the 90 degree shuffle, a player can often see what cards they are shuffling. The player has NO RIGHT TO SEE ANY CARDS IN A OPPONENTS DECK, but the 90 degree shuffle that is common for must people, allows the player to do this. (Shuffle a deck, tell me if you can't catch glimpses of the cards)
If a judge sees a player looking down while a 90 degree shuffle of an opponents deck is being done, they should give an immediate WARNING. This Sneak-A-Peak aspects of a shuffle is almost unavoidable when shuffleing a deck, thus that is the biggest reason it should be cracked down on.
Honestly, when they do it Everytime, Judge should caution the player for Gamesmenship.
When I read the floor rules, and I do believe the right to shuffle was only mentioned at the start of the game, but some how during the game is commonly allowed.