“Never spend your money before you have it.”
This reminds me of the “don’t count your chickens before they hatch” metaphor. I can’t tell you how many times I’ve calculated a bonus or paycheck before I’ve gotten it and then been disappointed after taxes, or on the rare occasion, pleasantly surprised at the outcome.
Even if your money is pretty consistent, you shouldn’t have already spent it (whether actually or mentally) because you never know when ‘stuff’ is going to hit the fan and that money you thought you had free, is suddenly no longer free. If you can avoid it, don’t even think of spending the money (except for regular budgeted expenses) until you have it in hand.