Box speed. Period.
It can take 1/3 of a stop over exposure, but there is so little difference why bother?
Generally manufactures know more about their own products than the customers do. Even more so with film.
I doubt very, very much an 0.3 stop difference is visible at all in print film. But it will be very obvious in transparency — up or down, to a trained eye.
With NPS 160 I routinely shove it in my XA and rate it at 400.
I shoot at box speed normally, but sometimes I want a slightly different look out of my film so I will underexpose and push it in development or make some slight change. Really it's all about what you are going for with your film. I am using film as an artistic medium so I don't necessarily want it to look "perfect".