i think its quite normal and natural that the fingers are arched more when playing on the higher strings.

i personally have discovered that my thumb position is far from "normal" i have my thumb not centered under my fingers as it should be. depending on where i am on the fretboard i have it either like one fret below my indexfinger or the higher i move up on the fretboard the more it reaches over the fretboard just like i owuld fret a note with the thumb. but since it feels comfortable and i dont have any problems with it, i´ll leave it that way. and thats basically what everyone should do. if you feel comfortable with a certain technique thats not "normal" and you dont have any problems with it, then stick to it. but when it starts to hinder you when playing certain things you´ll have to rethink it. i mean just look at marty friedmans picking hand. its completely spastic but for him it works!