It really depends on what type of implant you get. Saline implants are usually placed under
the muscle because the implant has less natural structure, and if you just put it under the skin you could see rippling. However most implants now are gel, because they have a more structured and natural feel to them, so they are usually placed over
the muscle (also less recovery time is associated with placing them over the muscle because you don't have to cut through the muscle).
Size is also a determining factor...if you stick a huge implant on a tiny frame, you'll very obviously be able to see the edges of the implant. My advice is choose a good doctor, who has experience, and you'll be fine. Always make sure to ask for real before and after photos during your consultation (every good surgeon will have plenty!) so you can see his work and decide if you like it.
This is my surgeons website, he's got a lot of good info: Breast Lift | The DeLorenzi Clinic - Plastic Surgery Kitchener, Breast Implants, Breast Augmentation, Breast Lift, Botox, Waterloo, Cambridge, Guelph, Toronto
and you can watch some videos here: Animations | The DeLorenzi Clinic - Plastic Surgery Kitchener, Breast Implants, Breast Augmentation, Breast Lift, Botox, Waterloo, Cambridge, Guelph, Toronto
Remember that everyones body will react differently to implants so there's no guarantees, but if you get a good doc your results will be good don't worry
Best of luck!