I used to place them loose in a non pocketed bra when I first bought breast enhancers. I kinda worked and at the same time it didn’t. Because they were loose, what I found was the breast enhancers would wind up shifting around and the boobs wind up being misaligned on my chest.
When I got proper breastforms, I’ve been using pocketed bras and have done so ever since. I do have double sided tape for the breastforms, but I already use up time to align my silicone hips correctly when transforming and I don’t want to chew up more time making sure my boobs are symmetrical. The pocketed bras by default will make the boobs symmetrical.