Why would you want to?
Raising temps in a mash tun can be quite difficult unless you can directly heat it under constant stirring, or indirectly using a RIMS or HERMS system. I generally use a plastic cooler mash tun, but sometimes heat directly (3-ply bottomed kettle on an induction plate) when performing multi-step mashing, under constant stirring while heating. Some brewers add enough boiling water to their plastic mash tun to raise the mash to mashout temps, but that reduces the volume left for sparging.
When you're batch sparging, you're draining the mash tun fast enough, then heating the lautered wort as soon as it hits the kettle, so a mashout really is not necessary. You can then sparge with ~180-190F water to raise the temp of the lautered grist to 170F to arrest further conversion.
Now fly sparging can take an hour or more, so a mashout before lautering is more warranted there.