There is no single answer to your question. It depends on what is most important to you, robustness or cost? If it's cost, specify an external 5V supply, keep it all of board, and if someone plugs in anything different, tough.
If you want a better user experience (i.e., better robustness), it will "cost". You can add a regulator to your board and specify an external 7V supply. That will keep heating to a minimum. Or spend a few more pennies and specify an LDO regulator (e.g., LM1117) and specify an external 6V supply, for even less on-board heating.
Or specify an external 5V supply and add protection circuitry that guards against overvoltages. No heating, but costs more money.
I would start the conversation by adding a Schottky diode and LM1117 LDO to provide reverse voltage protection and input voltages between 6V-20V or so (depending on how much current you draw). Board space requirements are minimal, costs are minimal, and you've already guarded against the most common (and unfortunately likely) user errors: reverse voltage and overvoltage.
: compatible with Arduino UNO, 24V operation, all I/O's fused and protected