I'm not sure; up/down is used for navigating links. There are three functions we ought to have (page turning, history, link navigation) but only two axis. So at least one of the axis must share functions. I guess this is very open to personal preferences, but I wouldn't want to lose any of them.
I see two configs I'd like equally:
left-backhist; right-pageturn; up/down-linknav
left/right
ageturn; up-backhist; down-linknav
but I lean slightly towards the first one because it shares more in common with the one shipped by default.