"Food grade" just means it won't contain residues of petroleum lubricants or other toxic substances. Beyond that, the various grades are a matter of purity, i.e. how much air is mixed in, and that's where the debate around here begins. Admittedly, even a tiny fraction of a percent O2 is many times the ppm total O2 ideally in beer to avoid oxidative staling, and if you've been exceptionally careful about avoiding O2 exposure up to the point of kegging, then this will be something you want to be aware of. If you force carbonate, you will add even more of this "contaminant" O2. However, if this is not a priority concern for you, then food or beverage grade is perfectly safe. It's your beer, your money, your choice.
(I put the purity of my beverage grade CO2 in the category of "stuff to be aware of but that I can't do anything about." If I'm worried about total O2 I'll address it at other points in my process.)