As I remember under the FAA rules you have to believe from any combination of reports or forcasts that the wx will be at or above minimums (visibility) at the time of your arrival. Sometimes companies try to push this one. Just be sure in your own mind that if you get the invitation to court over it that you can put forward a darn good case of why you thought it would be above minimums.
I haven't flown in the States for about a year so I stand well ready to be corrected. I personally like the JAA view of the two alternate option, just chose wisely.