Does she use a regular toilet when you're out? My daughters trained with a potty chair at home, but just used the regular toilets if we were out. We always found it easier to have them sit sideways on the potty, lean forward, and hold my hand-- that way they were over the seat far enough and they didn't have to touch anything except my hand. I really would not want to carry around one of those seat inserts unless it was absolutely necessary.
My son was using the regular toilet when we went to Disney in May, but we brought the potty chair with us for the hotel. He has a tendency to take 20 minutes to go sometimes, and it was also convenient for when he had to pee and some one else was in the bathroom. I brought pull-ups and he wore them to the parks the first day or two because I was paranoid, but we were there 12 days (plus two 23 hr car trips) and not one accident!