As one who is currently a student skydiver (11 solo jumps to my credit!) I can say that it's not too often that you look down - best to have head up to maintain the body in an "arch" so that you are stable.
Even if you were to look down and see an aircraft then avoiding action would not be guaranteed.
Having spent a fair amount of time at the drop zone over the last six weeks I am somewhat amazed at the numbers of aircraft which penetrate the DZ without calling. Granted that student pilots get lost sometimes but I have witnessed both military (3 basic trainers in formation) and civil aircraft in the DZ.
Anyone who penetrates an active DZ is exposing both themselves and parachutists to considerable danger! AVOID AVOID AVOID!!