For all scuba equipment, there is a 40% rule. If the piece of gear doesn't come into contact with gas containing greater than 40% O2, then no special equipment/cleaning/procedure is needed. If the gear does come into contact with any gas with greater than 40% O2, then it needs to be "O2 clean".
Technically, pre-mixed EANx of 40% or lower PO2 can be pumped into a non-"O2 clean" tank with no fear. There are usually laws and local regulations however so that EANx of other than 21% can't be put into non-marked tanks for safety sake, don't want someone unknowingly grabbing a hypo/hyper oxic mix and dying because they used it at an improper depth.
Partial Pressure mixing of Nitrox of any final PO2 can only be done into "O2 Clean" gear since at some point the gear will contact O2 at potentially greater than 40% O2.
All Nitrox tanks needs to have an "O2 clean" certification that is actually valid so that a Nitrox fill station can be assured that they can use whatever procedures they normally use, partial pressure mixing/premix/whatever safely.
Filling an "O2 clean" tank with "O2 compatable" or "hyperfiltered air" is permissable and should not void any "O2 clean" certification as it contains no more contaminants than EANx has. I haven't taken the mix course yet, but from what I remember from my Nitrox classes, the IANTD Nitrox manual and the SSI Nitrox manual both specificly mention that it is an acceptable practice. Non "O2 compatable" air is also specificly mentioned as not acceptable fills for a Nitrox tank. All the Nitrox label means is that you need to have the contents analysed and verified before use.
Taking any scuba tank, let alone an O2 clean Nitrox tank to a facility that you do not know uses hyperfiltered air and ask for an "air fill" without requesting to see their air quality testing certificate IS a foolish thing to do, for all you know they are using a rusty greasy bicycle pump manned by a Herculean muscleman. Heck, I have on occasion even asked for and have been shown the compressor/fill station/storage tank area.