I would say there are times that doing math with a primary key is a useful property (say, getting the Nth primary key (or so)) but if you are exposing it in an API I would say you would never even want a primary key projected in the first place.

A primary key is almost an implementation detail - a key that an API knows about something is one of many things that might point to this thing, might need to change, and generally might need a different representation (so don't make it your primary key.)

I also tell people to just use the bottom of any primary key space (when choosing monotonic stuff) but so many engineers just complain that they dont like the numbers (and yet many of them have had to deal with the migration a few years later so ... enjoy that I guess.)