The only ones in use that I know of, so far, are A0 and A1 for the A and B buttons, and A2 for the piezo speaker. There could be others used that we haven’t been told about, or will be added to the final design, like (hopefully) battery level monitoring. If you use a #define or const int to specify the pin to use, at the start of your sketch, and document it with a comment, then it’s easy to change later if necessary.
Why not write a sketch that reads many of them and displays the value returned for each? Put this in the loop() function with a delay of a second or so at the end. Then, pick one that looks good.
The wait for button approach, that @unclesporky described, also works well, especially for the Arduboy since most sketches will probably be games and have a press any key to start or similar action.
I think using one or both of the two methods above would work well enough. Writing your own function to gain just a bit more entropy would likely be a waste of time and code space.