Cheek implants are generally placed via one of two common incisions--the lower eyelid, or through the mouth and upper gum line. Hairline incisions are usually not used since there is the possibility of damage to the frontotemporal branch of the facial nerve (causing inability to blink or raise one's eyebrow).
The mouth incision has the highest likelihood for infection (because of lots of nasty mouth bacteria, and despite antibiotics), though this is still relatively uncommon. If infection DOES occur, the implant(s) must almost always be removed, the infection resolved, and adequate time allowed to elapse for scar tissue to soften before reinsertion of new implants.
The eyelid incision has the least likelihood for infection, but there can be scar tension on the lower eyelid which can affect the eyelid position, eye dryness, and require additional surgery. These complications are also uncommon, but the implant(s) can remain in place even if additional surgery is needed to address these lid problems.
The best approach is the one your chosen surgeon is most comfortable using, and the one which has the best success in his or her hands. Implants are always sterile at the time of insertion, unless contaminated during surgery (such as with mouth bacteria). Late infections are extremely rare, but I have seen one case in 25 years of practice, so it is possible. Overall, infection is one of the least likely complications; malposition is much more common, for example, though still unusual. Best wishes!