It iss just meant to allow air to bleed through. I'm not sure if it
really matters but it makes sense that it would work best at the highest
point(top).

porshapower@webtv.net (:\) :\)) wrote in news:23251-437844A3-966@storefull-
3334.bay.webtv.net:

Top, always.  It is to allow air to bleed through.  If the air
can't bleed, and a bubble forms around the thermostat, the
opening of the thermostat will be delayed and the engine will
overheat.

Voice of experience:
I rented a car from a guy that ran an independent used car lot
and rental car business.  I noticed that the car always ran
very hot at the start of a trip, then the temp would swing
between too hot and normal for the rest of the trip.  I spent
a lot of time thinking about it.  When I returned the car I
mentioned the problem. He said he has been trying to solve
the problem but can't. Told him to check the installation of the
thermostat.  A week later I was back visiting the lot and
he came over to  me and said the he fixed the problem.  
Thermostat was installed wrong.