This is a very personal decision that you and your surgeon will have to determine. As you probably already know, there are three incision placements: through the armpit (trans-axillary), around the areola (peri-areolar) and underneath the breast, where it meets the body (infra-mammary). There is also an incision in the belly button (trans-umbilical), however this is very uncommon because not only is it available solely to patients opting for saline, it also makes it more difficult to position the implant. Revision breast augmentation through this incision site is also impossible.
In my experience, the infra-mammary fold has been the most effective in achieving the expectations of my patients. I find that with this incision site, the implant can be placed below or above the muscle (sub-muscular or sub-glandular) in the most aesthetically pleasing position with the least risk of complications. I can feel any areas that need to be filled out with the implant better, so that it may sit more naturally. This is particularly important for the lower portion of the breast, to achieve a smoother and fuller contour. The other incision sites do not allow for this as efficiently or effectively. That being said, however, I do use all three incisions, depending on a patient's particular situation and preference.