I think that it's fine. If that's what you want to do. I think you learn a LOT about someone living with them even if you think you know them before you move in with them. I've learned SO much living with my bf. We only were with each other 9 times before I moved in with him!! It was love at first sight
It totally depends on the people involved...I think that living with together before marriage is one less 'shock' you have to deal with after you tie the knot!