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).
05/10 - Septicflesh
05/16 - Anaal Nathrakh
05/17 - Aborted (?)
05/21 - 05/24 - MDF XIII
05/27 - Cryptopsy