When selecting an eyepiece for use at the scope, though, the "around 300" is the operant figure.
Whether it's 288x or 287.5x or even 295x isn't really that important. I don't have enough eyepieces to choose one at every 10X magnification (though some CNers do ).
I have eyepieces that yield 388x and 493x, but I think of them as "around 400" and "around 500" because those are the significant figures to remember.
Plus, I regularly observe with others, and they don't want to know the magnification is 388.4x. If they ask, I say 400x, which also gives them an idea what to use
in their own scopes if they want a similarly-sized view.
I don't have a lot of scopes I have to remember magnifications for, so a rounding off is sufficient for me to remember the magnifications off the top of my head.
What is interesting, though, is that I tend to select eyepieces below 250x by focal length ("I think I should use the 12.5mm on this object") but above 250x,
I tend to choose based on magnification ("I think this object needs about 300x"). I know all the magnifications, but I don't select eyepieces that way.
Perhaps it's simply familiarity with the eyepieces and what views they offer.