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).
04/16 - Ringworm
04/19 - Trog
04/22 - Ghost
04/24 - Iron Reagan
05/13 - SLAYER