I have Tanis heaters on the IO-470's in my Baron, along with an EDM 700
engine monitor with no problems.

In an experiment I plugged in one engine and left the other unplugged
overnight in my heated hangar. I saw 42 degrees (the ambient temperature) on
the unheated side (both the CHT's and EGTS, which is a cheap calibration
check), and 65 degrees in the CHT's of all of the cylinders uniformly on the
heated side.

The Tanis heaters came with the plane, and since the hangar is heated, I had
never used them. However, I wanted to see how well they worked, which was
the reason for the original post in this thread.

My conclusion is that if the plane is going to sit outside below around
20-30 degrees for over three hours it may be worth plugging in the heaters.
They probably should be plugged in for at least 3-4 hours to get any effect,
depending on the outside temperature, and I recall Aviation Consumer had a
review of these systems in a recent article.