I let my developer solution "rest" overnight but the reason I do that is not the oxygen content or the Ph issue. With D76 and XTOL, right after mixing I see some undissolved particles floating around. I do not want them landing on film during processing and causing spots and uneven processing. In few hours, they will completely dissolve. So I mix, decanter into smaller bottles and seal, then 24 hours (or more) later, I use them for processing.