Just a bit anecdotal information here as I have just posted this at another forum so mostly cut/paste:
During the design phase, we tried a few different thermometer placements and types. No one could decide on what to put where, where to put the ports for probes, etc and all that. Then they started in about "Well we need it one size fit all and what if someone sticks a finger in there and gets burned?" Finally I gave up because, well.. There's a saying, "We don't all need to agree and we don't all need to have a vote, We just need a decision to be made." And those that were in that position, couldn't decide. The best I could do was the range on the bi-metal thermometer. Those are generally +/- 10% and certainly well enough for cooking. That final position (unless it's been moved) was where I drilled into the lid on my deck one afternoon.
Back then I used to have all kinds of thermometers and probes going in and out and monitoring temps. In testing we had hundred thermocouples glued to the WSM like a made scientist feeding into a DAQ (data acquisition) and spitting out temps over time into an Excel file.... A lot of it was information. Not entirely useful or helpful per se, but it was information.
I did an 18 hour shoulder on this past labor day. I started it at midnight, poured mop sauce on it for the first 2 hours or so but then went to bed. The temps dropped off about.. well I don't know when! I was asleep! But at 7:30am or so I stoked the coals and just let it sit around 200-250ish for the rest of the day, even going to the public pool with the family for a few hours. No temp monitoring of anything beyond the basic lid thermometer as I walked past it as I was doing other things.
Around 6pm I opened it up, got my $5 instant read Taylor which read 197.. Whelp, it's done.
Someone at the other forum asked about opening the lid and looking and the temps dropping. It's really not that big of deal. In the dead of winter when it's -9 and I open the door to take out the dog, my entire house doesn't drop in temp. The objects don't freeze -even those right next to the door. So certainly the internal temp of whatever you're cooking won't drop if you're checking things out for a moment.