I don't know if this helps but I think bike carbs are usually gravity fed (a handful of kPa, say less than 20kPa depending on how many Gs you can pull on a bike) so using a return line like that might help by keeping the fuel pressure minimal at the carbs. Certainly there'd only be a few kPa's at idle.

If you want a return line you'd probably want it to loop back after the carbs. Don't worry about check valves, the only place you might put one is in the carb feed to stop fuel draining back to the tank.