I personally would be very careful about having a joint account before being married. With marriage come certain rights and responsibilities. Before that, you are on your own. I would definitely check with an attorney what my responsibiltiies would be. Imagine, if your joint account is used for some illegal activity, even if you did not know, you are responsible.
In order to find the right answer for you, ask yourself why you want a joint account? Does it fulfill a need that could be fulfilled otherwise? If you just wanted to facilitate bill paying, then maybe you could set up a separate account just for that purpose.