I looked at the data again and found that my recollection wasn't too far off, but here's a little mroe that supports your experience. For 400 ASA Color Neg there was measureable but insignificant effects at 5 to 10 passes. Noticable effects were observed only at 25 passes or more. For 800 ASA noticable effects at 10 passes or more.
For most people this is well within the realm of "not a problem". I know for me it has never been, either.
I would ask when that testing was done. Imaging xray sensors are far more sensitive now than even one decade ago and the flux is much much lower.