BODY LINES !!!
it will stick out like a sore thumb if its lined up with the ground (and the ground is never level anyway)
What? Ground is never level? Anyway, some brands demand their logos be level and NOT follow body lines. They don't want any angle on their logo etc.
Many vans nowadays have lines that are not parallel and choosing which lines to match is a problem. Following the most obvious ones or the rocker panels may be the best choice.
Solution...Ask the CUSTOMER their preference when in doubt.