If your getting paid after the 20th makes you unable to pay on the 20th then you are illiquid. This may or may not be a big deal but it is strictly worse than the alternative.

For instance, what happens if you lose your income for whatever reason? Your paying irregularly might as well be proof that you will immediately be unable to pay rent in that case. Now, this may not in fact be true (perhaps you have lots of liquid assets but want the maximum amount of money invested at all times, and that was fine because your landlord didn't mind), but you can see why this is a red flag.