@deckingman Isn't there a risk of having the thermistor too far from the heat source to be an effective control loop?
Quick answer - yes.
Longer answer - there is also a risk that having it too close also screws up the control loop with respect to the bed surface temperature.
On balance, having tried both, I get much better control having the sensor closer to the upper surface. Warm up time is faster and temperature is stable. The only thing I've noticed is that it tends to run about 1 degree hotter than the set point. If that was critical, I'd simply choose a 1 degree lower set point.
Edit. When I said it warms up faster, I meant that the too surface of the plate reached temperature faster.