As I recall PADI recommendations is to have a backup plan equivalent to the primary plan. Which often is easiest with redundant devices and each buddy having the same or similar computer.

I think it’s very much not recommended to have going computerless as a backup. And the idea that you are safe to do so because your “within no-deco limits” is potentially dangerous, as your relying on the computer to tell you if you are still within that limit.

Many people go quite close to the limit. Personally I stay far away from them, but safety recommendations are the same for everyone.