If a heater isn't being used, it is better to block them off for nearly all engines. Looping them is the same as bypassing the radiator and will reduce cooling capacity. A heater valve in a stock system blocks flow in hot weather further supporting my point.

The only cases where a modified loop is needed would be for an EGR cooler, heated intake or heated throttle body.

And as a side note, for race only engines that might be run in cooler temperatures, I use a thermostat with two 3/16" holes drilled in the body. This makes coolant refills very easy ( don't have to wait for the thermostat to open ) and eliminates that cold slug of water / thermal shock from the rad first time the t stat opens from cold.