ro water for flush out necessary?

master kush plants after 8 weeks in flower in an ebb and flow tray in rockwool cubes are ready to flush. Is it necessary to use ro water? I just ask because it takes a lot more water to get through that filter and I thought if I could get away with tap water for the flush I would be saving precious water going down the drain. Or is the flush crucial to use the ro water? Thoughts on this?

Oh, the tap water comes out about 240ppm with a ph of about 8. The ro water has ppm of 0 and ph of 7.2

PH'd tap water is sufficient