It really depends on where you expect to sail. For example, I don't believe you're allowed to discharge either black or grey around the Great Barrier Reef
. Even if you're not going there, environmental laws will only get more restrictive in the future everywhere. If you're already replacing your tank you may as well make the provision to tank greywater. If I was doing it, I would put in a separate greywater tank - that gives you a little more flexibility.