No need to worry about keeping things running downhill. Pressure will get it all there. When pressure/flow stops, the check valve closes (it's supposed to anyway) and it's like putting your finger over one end of a straw. The fuel will likely stay in the line behind the check valve even if it is downhill to the tank.
If it's your main check valve (with a pill inside), it might be nice to have it a bit higher than the level of the fuel just because you won't spill as much when you change the main pill.
Returning fuel to the pump inlet is perfectly acceptable and can save you a bunch of hose. Sometimes the need to purchase fittings with return ports can cancel out any hose savings however.
Spud