If JonBenet had soiled her panties during the day at home she would most likely have left the main skid marks on those panties.
I am not sure if there were any soiled panties found on the bathroom floor, but if there were I don't think we can assume they were left there Christmas Day - they could have been there for any number of days, right up to the last time Linda H-P was there cleaning up.
I think JonBenet dropped the panties she was wearing on Christmas Day on the floor of her bedroom and got the size 12 ones out of her bedroom cupboard herself, and wore them to the Whites. I think she decided to do this because she did not want to go back to the bathroom to get a pair of her regular panties because her mother was in there, "balling" her red sweater and fuming and raging at her, and rather than have yet another confrontation with her mother, JonBenet decided to make do with some size 12 panties from her bedroom cupboard.