Difference between Phillips 00 and 000
000 is smaller and has thinner blades. 
00 is larger and has thicker blades. 

Using a tool that doesn't exactly match the screw means you can't get proper surface contact. This increases the chance of stripping the head of the screw on removal - especially if the screw threads have locking adhesive.  For assembly, the wrong tool makes it harder to torque the screw properly.

With a high quality tool and a screw that isn't too tight, you can sometimes mix 000/00 for screw and tool, but it's not ideal as the difference is significant for the tolerances in a MacBook Pro. If you're looking for an entire kit, this driver set is quite nice as well. You do get "a little play" any time there is a socket/bit connection; but for home use, iFixit sells great value and good quality tools.