I put in a 180 when mine specs a 197 but the fan doesn't kick on till 200 so it goes between the temps, or stays at 180 on the highway. Just gives a tad more leeway for error. I dunno, no problems so far 
180 is probably high enough to keep it closed loop. If you have low winter timps it might be a bit low. Remeber the rating on the Thermostat is the rating fully open not the rating of when it begins to open.
Somtimes there is a little odd problem that will happen. 160 Thrmostat may cause it to run hot. How can that be?!?!?!!? The thermostat restricts the flow. If the flow is too fast the coolant will not spend enough time in the radiator to shead the heat it picks up from the block. SB 400 GM enging is notorious for this. 180 Thermostats they are over heating. 195 and they will hold 190.
