For current to flow backwards, against the voltage generated by the DC/DC converter, you need some other energy source. If you have no such energy sources, then it can't happen. Possible sources:
- motors
- batteries
- connectors that someone might accidentally connect to one of the above
If you have any connectors at all, it's good practice to assume someone will connect them to something bad at least once. Putting diodes on them to clamp the maximum voltages seen here to the supply rails is pretty common, but then if these diodes start shunting excess voltage into the supply rails, where does it go?
simulate this circuit – Schematic created using CircuitLab
It's even common for CMOS ICs to have diodes like this internally on all their inputs and outputs for ESD protection. The microcontroller in your Arduino certainly has them. Are any of those pins accessible from external connectors that might be abused?
When excess voltage is encountered on these, it's dumped into the supply rails. If that current has nowhere to go, the supply voltage increases and things go boom.
The robust solution is some over-voltage protection, usually in the form of a crowbar circuit. Adding a diode to isolate the DC/DC converter from everything else will just protect the converter, leaving the overvoltage fault to destroy everything else.
