Hello.
We do not generally characterize our regulators for reverse current from output to input, though I expect it to usually be okay if the current does not get big. The basic architecture of all these regulators puts an inductor (basically a short) and a diode from output to input, so if you have something like a discharged battery on the input, there is nothing there to limit the current and so it could easily get big enough to damage the regulator (there’s no reason to expect a regulator with a 500mA current limit to somehow be fine with more current running through, regardless of direction).
The diode on the 5V line should protect you. Usually, that approach is not very appealing since you get some drop over that diode, but if you can tolerate that in your design, great!
By the way, be careful touching power electronics! They can get way hotter than burning (skin) temperature as part of normal operation, and if you are working with larger parts, they can stay hot for a bit even after you turn them off.
- Jan