Generally, you would want to have matched tubes so that you can avoid channel imbalances (i.e. one side is louder/quieter than the other). So you're essentially paying a fee for someone to test, and match up the closest pair of tubes for the set.

It's entirely optional, and you can always roll the dice, but if you happen to win the one in a million chance of getting obviously mismatched tubes, it would be more of a hassle to get a matching set, after the fact.