Varying rules across casinos significantly affect overall blackjack strategy, as different rule sets can either increase or decrease the house edge. Rules like the number of decks used, whether the dealer hits or stands on soft 17, or whether players can double down after splitting, all impact the optimal strategy. For example, fewer decks reduce the house edge and make card counting more effective, while rules like no re-splitting Aces or restrictions on doubling down limit strategic options.