Some say use a restrictor
some say use nothing
some say use a thermostat.
The fear is "thermonucleatic boiling" (whew), which is the formation of tiny localized bubbles in the hot areas of the heads. Once a blanket of small air bubbles form on the surface, the heat-conduction away from that surface diminishes. Having a pressurized system helps prevent these bubbles from forming and makes for better heat exchange.
So, if you believe in the bubble theory, run a thermostat or restrictor.
If you don't, run nothing.
Read the document titled "cooling system principles"; from Meziere's web site:
Or at the very least, this is what they have to say about thermostats:
For most applications, with the exception of drag racing, a thermostat is recommended. Most racers avoid thermostats seeing them as another part to fail, but their benefits far out weigh their stigma. A water outlet restrictor can be used in place of the thermostat if you prefer. In our opinion Robertshaw high flow (available from Mr. Gasket, Milidon, or Moroso), Stant Superstat or the highly reliable Cloristat used in Volvo 4 cylinder engines (fits Chevy V8s) are the best choices. The Robertshaw thermostat offers the least amount of restriction when fully open which is desirable for electric pumps incapable of producing high dynamic pressures. The thermostats main purpose is to bring the engine quickly up to operating temperature and maintain that temperature (see section; Recommended Operating Temp). The restriction at the water outlet will maintain an even block pressure (reducing localized boiling) during pressure surges produced by mechanical pump rpm cycles. Under deceleration the drop in pressure can allow the coolant to boil and in extreme cases this can occur in the radiator with the related expansion causing the radiators tubes to balloon.