I think it's okay to sit for some acts (not most) but it's hard to judge.
Example: I saw REO Speedwagon/Styx/.38 Special in June. Everyone sat for .38, stood for Styx, and about half and half for REO.
And when I saw Alice Cooper at the KS State Fair, everyone stood (to my surprise).
03/26 - STYG/Terror
04/01 - Earth
04/03 - MOTM Fest
04/08 - Destruction
04/09 - Goatwhore/1349