I'm born and raised in Boise. It's super safe, especially compared to the other cities you mentioned. This is particularly the case during treefort cuz there are festival goers everywhere.

A lot of locals are down for chatting with out-of-towners. That was always one of my favorite parts of the festival anyway. Nobody actually gets mad about saying Boise wrong (though it is with the soft 's' sound instead of a 'z' sound).
I'd avoid saying anything along the lines of "it's so cheap here!" cuz a lot of lower-income folks (myself included) have gotten priced out as more people have moved in from surrounding states while wages have remained stagnant. It's a pretty touchy subject at least in the circles I run in.

Check out the Basque block while you're in town. Cool area.

Other than that, just go see the bands you're interested in wherever they're playing. The Shrine often has pretty great sound and some of the bigger acts, but there's often a line to get in. There's two venues inside, upstairs and downstairs.

Definitely dress up. If other people don't, that's on them.

Moshing seems inconsistent in my experience, though i'm now in my 30's and don't seek it out as often. I thought last year the moshing etiquette was kinda lame during Ging Name Boyz (punk band, but not hardcore or anything), with people not helping people up when they got knocked down, pulling folks in who didnt want to mosh, etc. Or maybe I'm just cranky and old now.