A potential problem with pre-shaving the magnesium is that you are increasing the surface area that can oxidize (the white coating). If the container is seals and/or kept dry this will me minimized. But you may want to replenish periodically to make sure you have good shavings when you need them.

I considered pre-shaving onto Vaseline coated cotton balls. Has anyone done that?

BTW, it's not just an accelerant, magnesium burns super hot, much hotter than any other tinder you would carry. That's why it's effective in igniting your kindling. Magnesium ignites at
473 °C, 883 °F. and burns at 2200 °C, 4000 °F