the closer to neutral (7) the better
step towards the acid side and your H+ ions increase x 10 each step,
step towards the acidic side and your OH- ions increase x 10 yeach step,
The H+ and OH- will combine (negative attracts positive )and forms water, which is neutral and neutralies the mix.
So basically if you use a strong alkali solution (say PH10) and then rinse with an acid rinse (PH4), then in theory it will be neutralised post rinse, however it's the damage caused to the natural fibres in that moment between rinsing (as i say, chuck some bleach on your hair for 10 minutes and then rinse it off,) and also getting the concentration of the rinse bob on. Which obv isn't always going to work unless you use exact figures.
Whereas you can use a suitable woolsafe approved cleaner which you just spray on, and rinse with water, leaving it at the correct ph with little risk of browning or damage to fibres.