Experience has proved there isn’t a dead set rule when it comes billing order. The attendance usually varies depending on the situation.
The way I like to work out of town gigs is:
If I have been invited by a band/promoter to play on a bill I will request a supporting slot when planning the gig to avoid any uncomfortable or surprising situations. Gas isn’t getting any cheaper and, in my case, Canadian cities are generally 3-6 hours apart at best. So, if I am unfamiliar with the history of the promoter/bands involved, I like to research the likelihood the event will reach a satisfactory attendance (subjective of course). If the other bands on the bill are generally unknown, I usually reference the number of people that clicked ‘going’ on their previous Facebook invites. No guarantees, but it’s a useful reference. It’s also good to check if they’ve received any internet media attention.
I only really agree to close out of town shows if there is a bit of hype behind my band coming to play, or the event is reputable, or the promoter is offering a guarantee. If you don’t know the promoter or someone that has previously worked with him/her, get the fee in advance.
Basically, there is nothing more to be had by playing a poorly attended out of town gig as there is an empty room on your home turf.