We've always played Shield Master can use his bonus action to Shove or Knock Prone before making his attack rolls. Without allies, it is nearly a pointless feature if you play it any other way. Imagine a fighter with Shield Master in a one-on-one contest. If you force him to make the attack rolls prior to his bonus action, he gains no benefit for knocking his opponent prone after the attacks are made since the foe can simple stand up for half his movement on his turn. By allowing the bonus action to occur prior, he can then make any attacks with advantage. This gives the feature purpose IMO. It is also much more cinematic and follows understandable tactical actions.
On a similar note, our DM allows the off-hand bonus action attack to also take place before the main-hand if the player so chooses, or if they have the extra attack feature at higher levels, the off-hand attack can happen between the two main-hand attacks. It is
illogical to assume every time the character attacks they MUST strike with the main-hand first. We are well aware that is not how the rules intend, but it makes more sense to allow the attacks to be more fluid. For instance, suppose you have a shortsword in one hand and dagger in the off-hand. You can throw the dagger in your off-hand as a bonus action at a target, then move to attack another target, and use your attack action against the second target. I will go further in stating an example that would really throw more "pure" rules players out of whack:
You throw a dagger from your off-hand as an attack, move and draw another dagger as part of the movement, strike with the dagger now in off-hand as your bonus action, and then conclude your attack action using "extra attack" to strike with your main-hand weapon! Crazy, huh!?!
Of course, we DO play with declarations of actions because we believe a six-second round is simply too quick to not have declared intentions, but that is our preference and another can of worms.
P.S. The fluffy bunnies are still waiting for someone to pet them. Have a heart, pet a fluffy bunny today!