Pixie has a point, doing what works for you will at least keep you playing. I personally would recommend however that you try to learn to use your index finger as your primary strumming finger. If you eventually want to move past simple up, down type strumming to fan, triplets, split strokes, finger picking, all of these will be more difficult if you don't use you index finger. If you click the Learn To Play Ukulele link at the top of the page and watch Aldrine's first video he shows you the proper index finger strumming technique, and it's easy!