AggieSarah01 said:
I never can tell if I have it placed right in the middle of the chicken breast. I will think it is done, and then I try sticking the thermometer in a different place and it is sometimes 10-15 degrees cooler.
I feel like this is the real crux of your question. The middle of the meat gains (or loses) heat as a single mass. Scooting over 1/2 an inch on a chicken breast or even 3 inches on a large roast won't matter. It's a big brick and all changes temperature at the same time.
However, poking new holes causes juices to move and give improper readings. Also, the two thermometers may not be calibrated identically. Lastly, the "instant" reads often take a bit to actually read.
I do my best to place my leave-in probe well, and then leave it and trust it. Don't second guess yourself.
The only caveat to this is bone-in cuts. Those bones don't gain heat at the same rate as the rest of the meat, and juices can move freely along the bone perimeters. Sometimes the meat right by the bone will be much more done quickly, and other times it lags the rest of the cook by 5+ minutes. This is the situation for two thermometers. Instant read to poke the bone, and probe for the middle of the meat. Still, trust your tools.