Lakeroadster, I agree on the thermal properties.
Here are some values which seem to vary depending on whose book you look at.
steel is 7.3*10-6 In/in/deg F
Stainless is 9.6*10-6 In/in deg F
However the yielding length of the fastener is only about .375 in. (threads don't count). I don't have exact facts but I would doubt that the fastener would see greater than 300 deg F on the head assuming aluminum heads.
The difference in length is about .00082" stainless vs.
or about .0003" rounding off. Pretty small amount considering that there is a compressable gasket. Also an alloy steel bolt has a greater yield value than a soft stainless bolt. So the SS bolt will act as sort of a spring if not over tightened. This is probably what the Belleville washer idea was to promote. Keep in mind that SS can have up to about 40% elongation vs about 20% for steel. Also there are other grades of SS which approach steel fasteners in yield specifications.
This is really getting into fine stuff and considering that the threads may not be clean, the bolt hole is not perfect, the flange is not dead flat on either side, and it is usually next to impossible to accurately torque a header bolt due to tight quarters.
I hate using SS fasteners on street cars however this is one area for which I give in ....but I use lots of Never-Seize and I am very careful about cleaning threads and other surfaces. Once burned, twice smart haha