This is referring to our 32 oz. Blue Devil Head Gasket Sealant (commercial grade).  Answer: Yes IF you want to improve your success rate.  You have about an 85% success rate  for most vehicles.   If the thermostat is difficult to remove---   Ex.  Land Rover Discovery (see  image), then most people "take their chances" since the stats are still relatively high. 

If you are realistic about the fix and not trying to perform a "miracle"- the success rate is 90%. 

NOTE:  Please do not confuse the Commercial Grade Blue Devil with the 16 oz.    -You do not have to remove the thermostat for the 16oz version (less concentrated, formulated for 4 cylinders and V6s).  -Success rate is currently 100%- but it's been out less than year so our fingers are crossed!