If you put a "s" the hose it will have some to flex in case of vibrations or something ,like a rock, jumping up and smacking the hose. If the hose is straight from connection to connection and something hit it it ay pull it off the fitting. On some of our a/c we have rebuilt or built we actually use a loop in the line to soak up vibration and jolts. Also you may want to use a hydraulic type hose since they are so much tougher.
The approved fuel hose is plenty tough, so don't use any other type. The compound in fuel hose is made for fuel, use it. Not necessary to leave a whole bunch of slack in the hoses. They don't shrink very much. Leave them over-length at the tank nipples until the tank is bolted up in there, and cut to length with a bit of slack & hook'em up. I've never seen S curves in fuel hoses. Maybe a factory thing to speed up production on the later Elcos ?