Hi, the reason you  use a degree wheel on the crankshaft to set the timing , and finding TDC is because , when the piston moves just 0.016mm at TDC the crankshaft rotates 4 degrees !  
Setting TDC  using a "piston stop" and degree wheel method is an accurate way to be within  0.004mm ! I'll bet  you cant do that with a screw driver down the plug hole !
cheers