The release notes for Adventures in the Forgotten Realms have this to say: "While playing Planechase, rolling the planar die will cause any ability that triggers whenever a player rolls one or more dice to trigger. However, any effect that refers to a numerical result will ignore the rolling of the planar die", and that includes effects of the kind found in Barbarian Class and Pixie Guide.
So so you then get to choose which result to go with or what?
Unfortunately, what it means to "ignore the lowest roll" of a planar die wasn't clarified sufficiently in the release notes. If this is something you care about, you should ask Jesse Dunks, the rules manager.
EDIT: Strike out certain text after comment 5 was posted.