5 grains is the bare minium. You can shoot that or lighter, but your bow will wear out very quickly. The bushings in the cams, the limb pockets all your accessories will become rattle buckets in a short period at 5 grains per pound.
The next question is why? If it's for 3D target shoots and you are a very serious competitor, getting 285 fps at 60 lbs draw would be very useful. If you are casual 3Der or hunter, it is not worth the noise, damage and lower penetration that comes with it.
400 grains at 64 lbs and 28" draw shoots about 260 fps on most bows. This is a very comfortable hunting set up. A bit slow for serious 3D work, but if you are good at judging distance it isn't a problem.
If you are shooting for 12's then 5 grains per pound is useful, but unless you are using a magnified scope, you can't see the 12 ring and going lighter is a waste of time.
Cheers,
Pete