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).
07/02 - Celeste/Primitive Man
07/06 - Jungle Rot
07/13 - Black Breath
07/16 - Psycroptic
07/19 - Danzig
07/22 - Melt-Banana