The "dropping process" after breast implant surgery typically occurs over the first 6-8 weeks during which time the tissues are stretching somewhat to accomodate the implants. There are alot of factors that influence the rate and degree of dropping such as the elasticity of the tissues, pregnancy history and the weight and size of the implants.
Typically an underwire bra is not encouraged immediately after surgery to avoid irritation of the suture line and to allow the natural, normal early implant settling/positioning to occur. The dropping process should be final well before 6 months unless there are other factors such as pregnancy or large weight fluctuations. Wearing an underwire bra at that time is a personal choice and completely acceptable but will have little impact on implant positioning out of the bra.
Web reference: http://www.drlevens.com