what about change for 50?

It would be fewer bills than without the $25, but the greedy strategy would still work for both (without, you'd get 3 bills: $20, $20, $10, and that's the optimal). Whereas change for $40, the greedy strategy will give you 3 bills: $25, $10, $5—which is more than the optimal 2 bills.

change of 50 in two bills and change of 40 in three, is less optimal than change of 50 in three and 40 in two?

It's not about comparing the number of bills vs. each other—it's not surprising that adding an additional denomination lets you make change in fewer bills. It's about whether the greedy algorithm for making change gets you the optimum number. Currently it does, but if you add a $25 bill, it would not.

Currently:

$40: greedy: $20, $20

  optimal: $20, $20

  is greedy optimal: Yes

$50:

  greedy: $20, $20, $10

  optimal: $20, $20, $10

  is greedy optimal: Yes

Including a $25 bill:

$40:

  greedy: $25, $10, $5

  optimal: $20, $20

  is greedy optimal: No

$50:

  greedy: $25, $25

  optimal: $25, $25

  is greedy optimal: Yes

Given that we don't have $40 bills, I'm not sure this is a problem.