Well, you can request the models to remove the straps of their bra (if it is removable) usually i go for strapless bras ... i can't stand the discomfort of the straps.
Or ask them to purchase bras that they are able to remove and put on the straps anytime.
Or maybe they did not know that the straps are removable, you could try asking them to test it out. Most of the bras on retail have that functionality.
This applies to dresses as well, so there is more flexibility involved.