While I completely respect each patient and surgeon's right to choose how they do this, my personal opinion is that firmly committing to an implant before surgery is a mistake.
The belief that an implant's appearance in a bra or computer simulator is going to be the same after it is placed under your muscle, breast tissue, and skin is (IMHO) what leads to so many women (up to 1 out of every 10) having another operation within the first year because they are unhappy with the size of their implants.
Think about it- are you pursuing breast augmentation because you want to have a certain letter on your bra, or because you want to achieve a certain appearance? If your breasts look exactly like what you wanted them to look like, are you going to be unhappy because the number on the implant is not what you thought it was going to be?
Choosing the implants before surgery may give a patient the sense of control and empowerment and involvement in their care, and it definitely allows the surgeon to absolve himself of responsibility if the patient says they are unhappy after surgery ("you picked them"), but is not the best way to achieve a happy patient.
The fundamental truth is that your skin and breast tissues (to say nothing of your chest muscles) are different that those of any other woman, and that those tissue differences will absolutely and necessarily affect the way that any particular implant looks inside of you. To think that a computer simulator or an implant outside of your body can accurately predict these infinite variations in people's anatomy is wishful thinking at best.
I'm not saying that these are not useful tools- and may be a good way to "narrow it down" to a range of implant volumes that may make you happy... But the bottom line should ALWAYS be the appearance you want to have when you're done.
Find a skilled and talented surgeon Certified by The American Board of Plastic Surgery who you like and feel you can trust and carefully communicate your goals for appearance to them. If they ask you to play with implants in your bra or put you in front of their computer simulator, fine. But make it clear to them that you want them to choose the implant based on your goals for appearance after the effects of your tissues on the implant's appearance are clear during surgery.
Your chances of being happy are much higher.