Deck checks must be performed at all Competitive and Professional Rules Enforcement Level tournaments, and the Head Judge has the option to perform deck checks at Regular Rules Enforcement Level tournaments. At least ten percent of all decks should be checked over the course of the tournament.
Deck Checks are an important deterrence to certain forms of cheating, discouraging players from doing anything dishonest with their deck, since deck checks are going on. Deck checks should be frequent and visible. However, if you find yourself doing deck checks on more than 10% of the field, you might be unintentionally trolling the players. While Deck checks are great for higher stakes tournaments, they are a fairly intrusive burden at Regular REL. Be very wary of using this option.
A full deck check should not be performed if a player has drawn an opening hand and potentially made mulligan decisions.
Judges can still do a deck check after the opening hand/mulligans have been made, but the cost of a judge error is significantly higher in this case, and is typically not worth the risk unless there is a very specific reason for checking the deck at that exact moment.