When you say runs at 70fps, do you mean refresh rate? That's separate from the physics step used to calculate things that must happen at fixed intervals.The default is 60 which is trivial, unless you're doing something insane - attempting 120 is fine and if there are skipped steps it will just slow physics calculations that may effect what's seen on screen. As you're already asking the engine calculate physics above its default, there's margin for error.
As with anything, your individual game results may vary and you're just going to have to try it if it is an important factor of your game.