It has its good side and its bad side.
On the good side, all your clients are billed on the same day / same cycle. No wondering about "when is such-and-so's next bill due": if they're paying monthly, it's due on the first, same as everyone else's.
On the bad side, your cash flow is once again fixed: high at the end/beginning of each month, with a 28-day dry spell in the middle. Also, if you get a client who wants to rearrange their billing date so that it falls on, say, the 10th of each month, to make things easier for them, you might not be able to accomodate them unless they're billed in a separate manner from any other client.