I've always heard it was bad too, but my dad has a street rod with 31.50-15 bias ply Mickey Thompson Sportsman Pros on the rear, and he replaced the fronts recently with common radials (they were formerly Sportsman bias ply front runners) and noticed no difference in handling or drivability. Personally I wouldn't do it, and told him that too, but in his case, it doesn't seem to make any difference...maybe it's because the rears are already so much wider than the fronts anyhow?
Personally, I'd order up a pair of front bias tires too, it'll look cooler overall, and take any question out of the safety of it.