Long answer: I like mixing in general, but I'm not a fan of chrome and nickel together specifically because they are so close but so not close; one is slightly blue, the other slightly yellow. To me it looks like it was just accidentally mismatched, though I think it's worse with polished nickel and chrome. That said, I do have polished nickel hardware everywhere in my bathroom except a little of the exposed plumbing with my pedestal sink and toilet, which is chrome. Maybe that's why it looks accidental to me, because it was an accident in my house. I plan to eventually buy fancy nickel plumbing parts for the lines, valves, etc., but two years later it hasn't bothered me enough to drive to Sacramento to actually do it.
Another explanation of my dislike might just be my general dislike of the coolness of chrome. I much prefer warm tones. So that's just personal preference.
I have mixed brass and ORB all over my house and like it. I plan to do a mix of ORB or brass and polished nickel, I think, in my kitchen. I like a collected feel. However, I can imagine that in a simpler, more modern space I wouldn't want the eclectic feel, and I'd do high end simple fixtures in all the same finish just to keep visual clutter from ruining the look.
I would do chrome. I see a toilet paper holder as more of a fixture like the shower head and towel bar. The cabinet hardware is small and the lighting finish is only really noticeable when the light is off. I'm not a big fan of mixing in a small space like a bathroom unless it's intentional and noticeable (ie. polished nickel fixtures and aged brass lighting).
i don't mind chrome and brushed nickel being mixed as you described. i'm with FoxInFiji in that i do see the toilet paper holder as more of a fixture like the shower head and towel bar, so i would go chrome.
however, i happen to have a light fixture that has both brushed nickel and chrome in its design for our master bath, so it allows for both to exist in the space by making some sense of it. our other rooms have all chrome going on.