I wouldn't cancel F1 just in case if anything goes wrong with AOS. If that happens at least ur wife can try to get back to F if she needs to. I don't know the whole thing about this but it happens to me. I didn't expect that they would deny my I-485 app, so I let my F ran out. But I guess we have to expect what unexpected. When they denied my AOS, they count back since the first day that my F ran out. I thought that they would count from the day of the denial, but I was wrong. That makes me out of status for about 4 months already. And by all mean, I was told that the more day I am acruing for being out of status, the more trouble I can get into.