This is a very individual decision, and obviously one that changes drastically with belief system and culture. Personally, I believe there is nothing wrong with living with someone before you're married, though I generally have a problem with it if two people haven't already made a serious commitment to each other. That said, if casual daters want to meld their home and finances, there's nothing anyone outside of the relationship can say about it.
I lived with both my husbands for a time before marrying them -- which was sufficient time for me to know that I shouldn't have married the first, though I was too inexperienced and stubborn to not go through with it. With my husband, we may not have lived together first if circumstances had been different, but we lived far apart and he moved here shortly after we got together. It just plain didn't make sense for him to get his own place when we were already planning on forever, and we were married a few months later.