Even with the heater flow turned off there is enough turbulent flow and heat conduction that it is a good reading. Done it before, everything is good. There is more error in the senders that that tiny location change. You could also argue that the stock temp sensor isn't valid because it is only getting water temp off the driver's side head and the passenger side head mixes in after the sender.
I saw the OP was Minnesota so I didn't suggest this, but it works fine in AZ. Tap the heater water off the back of the intake and use the 2 port up front for the gauge and computer. The downfall of this is the water isn't as hot as it hasn't traveled through the heads yet. But you don't need that much heat in AZ typically.