Depends, on the factory tolerances I suppose 59 to 51 degreee BTDC @ .006 Lift Intake or 101 to 93 degree BBDC @ .006 Lift Exhaust.
If on the intake side 59 or 51 is a preferred number but engines fall all over the range. Twisting to consistantly get your engines into the preferred number of legal levels could net you a gain over engines not at the optimum number. I dont have acess to a great number of engines but I can imagine if they see a need to keep the rulebook so open the factory isnt great at holding the tolerance perfect. (Most likely on the crankshaft gearing)