Maureen,

I think you may not need to be that cautious about overheating unless there
are definite symptoms.   
In our '90 Westy (only 80k miles), I can sit in traffic for many minutes
before the temp rises much, and it is always well-behaved when the fan
comes on low speed even with the AC on in 90 degree weather, so if your
radiator is in good shape, you shouldn't be having any problems and
shouldn't have to use the heater at all (except in winter, of course...).

We've also got an 87 that currently has a partially blocked radiator (yeah,
I've got a replacement and I'll change it when I get around to it...), and
on it, the temp will start to rise when I'm stopped for longer than ~2
minutes in hot weather.  I watch the gauge carefully, but when the cooling
fan comes on at low speed, I've found that if I raise the engine revs above
2000 RPM, the additional water flowing through the radiator seems to keep
the temp from going higher.   When my wife drives it she turns on the
heater when it heats up, though. 

Incidently, there is a H I G H speed to the radiator fan that sounds like a
low-flying airplane when it comes on (I've only heard it once on either
van), so don't be alarmed if it does.   

Dave

87 Wolfsburg 
90 Westfalia