I'm getting a lower facelift and neck lift in a few weeks, my doctor has advised using hydrogen peroxide on the stitches?

Hydrogen peroxide on the incisions is helpful to help keep a wound free of dried and crusted blood. There is a theoretical possibility that it can affect the healing of a wound, but as long as it is used in moderation, it should not affect the result of the healing. For use along an incision, there is no reason why peroxide cannot be used to help keep the wound clean and free of blood. 

If a wound has dried blood, or is otherwise very crusty, hydrogen peroxide (H2O2) can be useful for cleaning in limited amounts. However, in general H2O2 is a wound healing deterrent due to release of free radicals in the cleansing process.  The best cleanser for most situations in saline, followed by an emollient such as aquaphor.