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).
07/22 - Melt-Banana
07/26 - Forever Came Calling & Primitive Man
07/30 - Warped (?)
08/05 - Knockout Kid
08/07 - Fireworks (?)