well there IS a bit of a gamble...
breeding a pied to a pied will get you 100% pieds, no gamble
breeding het to het SHOULD get you 25% pieds
breeding het to pied SHOULD get you 50% pieds

breeding het to het, you may not get any visuals, but then again you may get lucky and get more than 25%, that is the gamble (assuming the breeder is trustworthy)
breeding het to visual, you would have to be very unlucky to not get any visuals, but I suppose it could happen...