Return lines on fuel systems for engines with carburetors and engine-mounted mechanical fuel pumps weren't/aren't really necessary. When the engine is off, the fuel pump is also off.
Return lines are arguably advisable on systems with electric fuel pumps. There's a possibility of the pump continuing to run when the engine is off. Even at relatively low pressure, there's a possibility of fuel being force out of the system. The result would be dangerous pools of fuel which could ignite.
Return lines (and pressure regulators) are definitely necessary on fuel injection systems. Significantly higher fuel pressures are needed to make the injectors work properly. This makes it much more difficult to contain surplus fuel.
