If you plan to ever slip your print economically into a premade frame, then about the only "rule" that might apply is that the border should be either more or less wide than the moulding, but never the same width as the moulding.
Holding this thought, If you plan to sell your prints you can pitch the ease with which they can be framed thanks for your border sizing, without further ado about mattes etc which for the most part are missing from framed photographs that sell for real money these days. Just slip it behind the glass without any fuss and call it a day.
Frinstance here's an example of the last tolerable photograph that sold for a truly significant amount of moolah. That white stuff is just bare print.
And you ever give a gallery bare prints to sell for you, give those prints a nice wide border so you can later cut off the fingerprints of the unwashed boobies that will riffle roughshod through your art, unconstrained by the gallery staff.