I have ADS-B both in and out. The traffic info I get on my screen shows other aircraft altitude relative to my altitude with 100' accuracy. So I recommend if you are planning eventually to have ADS-B that your altitude reporting is accurate to at least 100' rather than 200'. As you can see, if my altitude was off by 200' and the other guy coming at me was off by 200', the total error on my readout could be as much as 400'; kind of ruins the usefulness of the whole thing. I say be as accurate as you can; its not that hard when you are in the building phase.