Surgeons often recommend bras around the time of an operation. But, wearing a bra on a daily basis on native breasts cannot prevent ptosis.
A very tight constricting bra theoretically could slow down the normal process of implant dropping. In my practice I encourage patients to wear only a soft supportive bra and when at home if comfortable to go without a bra. I also encouraged patient's to do breast implant displacement exercises to help the dropping process.