What is the best type of grafts to use for a sliding genioplasty "step off" and "labiomental fold"?

Re-shaping of the bones can be sometimes accomplished without the need for bone grafts. In cases that require grafting I would use a bone graft consisting of cadaver bone that comes in the form of a bone putty. I can be shaped and formed into the defect and generally gets incorporated into the existing bone and prevents the need for a donor site. This is an office procedure and does not require general anesthesia.

While just about any implant or graft will work and they all have a low rate of complications, any time you can achieve the desired augmentative effect without using an implant that is always best. I find that using allogeneic corticocancellous bone chips/particles works well for the augmentation, is applied like a bone putty, allows tissue ingrowth and much of it is replaced by bone ingrowth. While your own bone is always best few people want to have a bone harvest site for an elective surgical procedure. (although some do) 

The use of materials to 'blend' the boney cuts and contours into the rest of the chin/jawline is a very good idea and usually associated with an artistic genioplasty. Generally, one wants to use a material which you can customize, place accurately, not shift and 'feel' 'even'without bumps etc. Gortex works best since one can shape it as you need it. Bone materials are hard to shape and taper and always undergo change due to natural body remodelling which can leave defects over time.