Alright:
Tested the Iron, Steel (horned version too), Elven, Orcish, Hide, Scaled, Fur, Imperial (light and officer version) Glass, Dragon (both of them), Stormcloak Officer, and Falmer helmet, Mage and Monk hoods and hooded robes, and no clipping occurred with the longest straight hair.
Some of the helmets like Imperial, Steel, Iron, Scaled, etc. show some hair under them (normal thing), others like Glass, Elven, Stormcloak Officer (Dwarven and Ebony too), etc. display no hair at all (normal thing). All hoods are alright, too.
Tell me if I missed anything.
Edit: Also tested hats and caps. All seem fine. The only problem with a few hoods is that they show tiny tips of ears. I think it's only for one ear, and not very noticeable either.
I must say, this is the best favor anyone has ever done for me through the forums. I cannot thank you enough!
I believe that if all of these pieces of head wear tested negative for clipping, then it is safe to say that the longest straight hair should be as safe as any other safe hair style in the game.
Really, thank you.
For fun some time to all else who may be reading this, and have access to console commands on the PC, and thereby can change their character's hair at anytime, I strongly suggest switching to the long hair with 2 braids on each side of the face.
Notice how severely bugged this hairstyle is. It literally clips through every hood, hat, cap, and most helmets.
It is really disappointing for me, because this hairstyle is my favorite one in the game.
However, thanks to Arcona Cor, I can choose my 2nd favorite style, without fear.