Some folks would say don't worry about it, but I am nutty about having the same brand and model on each end, so I certainly won't condone mixing bias and radial....BUT if you must mix them the radial must be on the rear. That has always been the rule going back to the days when radials were rare and expensive for cars, and folks would buy two, and it was said if you can only get two, put them on the non steering axle. Part of the problem for a bike as I see it is different braking characteristics between bias and radial. Also, curve taking response is going to be different too...one end might stick like glue while the other wants to wander. Don't need that. But if you ride and can't feel anything quirky about it, go ahead and wear them out, and then treat yourself to a matched set of whatever suits your fancy.