Matt's answer is right on.

Ah, yes, the annual "Is it necessary to pre-heat?" thread. Soon to
be followed by the "Should I leave it plugged in all the time?"
discussion!

Just to get the ball rolling, we pre-heat Atlas whenever the temp is
below 40 degrees F. And, yes, we leave it plugged in all the time.
("It" being a Tanis oil pan and cylinder head heater with a quilted
nose cowl wrap to hold the heat in...)

Here's a pic of our cowl wrap, custom made by Mary from an old nylon
sleeping bag and a bunch of velcro:
http://tinyurl.com/2oj9g7

We fly at least weekly, so I don't worry about condensation. IMHO,
plugging in the heater immediately after shut-down should help to
prevent condensation from developing, since the engine never cools
back to ambient temperature before it's started and brought back up to
temperature at the next flight.
--
Jay Honeck
Iowa City, IA
Pathfinder N56993
www.AlexisParkInn.com
"Your Aviation Destination"