I don't know. I didn't tak my girlfriend on a tour a few years ago and I am still paying for it.
I think it has more to do with whether or not the person you are bringing with you gets along with everyone and does not mind being a huckster for your merch. On that same tour we brought a friend of ours to take care of that stuff (We called him our "wrangler") and we had tons of problems mostly because doing his job hurt his pride. I think my girlfriend would have done a much better job.