I have used pennies, which I think run 9 to the oz. but I have gone to to BB's because I can make weight changes in smaller increments. Generally I do build in an adjustable weight box. One time I didn't and got too much weight in the outside wing. The airplane would hinge badly on square corners, so I added weight to the inside wing until I got the hinging down to a very small amount. Not an optimum solution.

Jim