It just depends on the chassis design and where everything has to go. On some drag cars the rear end is higher than the crankshaft so the driveshaft goes uphill. You also have suspension travel to account for and you have to figure out what the pinion is doing during acceleration as well as suspension travel. The rear end usually moves in an arc rather than just straight up and down. You might want to mock up the rear suspension with light springs and move it thru the travel just to see what happens with the driveshaft and pinion angle.