Breastforms that use the Hollister adhesive to glue onto the chest will most definitely hold on without the use of a bra. I should know from personal experience. 
However, when first gluing them on, I do wear a bra for the first 30 to 60 minutes so that the glue is allowed to take a set. After that, you can take the bra off and enjoy the natural feel of them tugging at your chest and the jiggling/bouncing of them. 
Perhaps it is an obvious thing for me to state but do shave hair from your chest where the breastforms will be glued. If you don't shave, you'll be wishing you did when the time comes to pull them off.
However, when first gluing them on, I do wear a bra for the first 30 to 60 minutes so that the glue is allowed to take a set. After that, you can take the bra off and enjoy the natural feel of them tugging at your chest and the jiggling/bouncing of them. Perhaps it is an obvious thing for me to state but do shave hair from your chest where the breastforms will be glued. If you don't shave, you'll be wishing you did when the time comes to pull them off.
Anne