It's not about freeflying vs. belly flying, freeflying is being able to fly at any angle.
From a safety perspective, you really don't have to be a fantastic belly flyer to safely attempt freeflying. The USPA student program probably meets all of the requirements for safety since all you really need to be able to do is turn off of jump run, get stable after losing it, and track away.
However, there are some significant advantages to belly flying. First of all, belly skills pay the bills. If you ever want to work as a camera flyer, tandem, or AFF jumpmaster, you'll need to be able to fly on your belly. Also, freeflying is about flying your body at any angle, so to be a good freeflyer, you need to be a good belly flyer as well as a good sitfly or headdowner.