Of the the three processes: Reverse Osmosis, Distillation and Deionisation, the deionisation will give the purest water which is also closest to PH7.

Water is what is called "amphiprotic" which means it reacts with itself. What that means is that if you remove the ions, which is one of the things a deionisation unit will do ( as well as removing trace elements ), it will, all by itself, become reionised to a certain extent until it reaches an equilibrium. i.e. the PH will change. PH means Power of Hydrogen ionisation. The trace elements will not reappear though so it will still be very pure water and the buffering in the developer is more than adequate to cope with the small amount of PH change due to water not being at exactly PH7.

So having deionised your water, if you let it stand for a day or two, its PH will change and may well become useable by your processor, but maybe not depending on its sensitivity.