P-J saved me a few bucks by not putting a PID/Temp probe/fitting/etc and all the wiring, parts, etc to make it all work, just to monitor mash temps. I have a dial thermometer that gives me a look at the temp of the wort coming out of the MLT on its way to the HERMS coil.
I honestly think that even watching that can add confusion (maybe even stress). If the temp is too low, I have a tendency to want to crank the temp of the HLT to pick up more heat from the HERMS coil. Then I overshoot as the hotter wort is dumped on top of the mash and it takes time for that warmer wort to get to the bottom and pass over the thermometer again.
If you're doing a HERMS or RIMS system, trust your equipment. If you use a HERMS system, set your HLT a degree or two above your mash temp and leave it alone. It will never overshoot if you let it do its job. If you can do your part to use the proper temp on your strike water, the rest of the mash should be set it and forget it, especially if you're recirculating.
P-J will help you get where you want to go and he'll help you avoid spending money on stuff that's not necessary, unless you insist on it.