Rounded values are usually fine. In practice, depth of field calculations are only approximations, so the tiny difference between f/2.8 and f/2.828 is not meaningful.

Several factors in DOF are already approximate: actual focal length can vary from the marked value, effective aperture changes with focus distance, and DOF itself depends on an arbitrary circle of confusion threshold rather than a hard sharp/unsharp boundary. Because of that, DOF and hyperfocal results are inherently rough guides, not exact measurements.

So for a calculator, using standard rounded f-stops is reasonable and matches what photographers expect to see. If you want, you could still compute internally with exact values while displaying rounded ones, but users are unlikely to see any practical difference.

If your goal is usefulness rather than mathematical purity, factors like circle of confusion assumptions—and possibly diffraction—matter far more than whether you use 2.8 or 2.828.