The school of thought is, if you have it torn down this far it's better to play it safe and replace the bearings to make really sure they are OK and to get the best tolerances when the assembly is complete.
If you don't use a solid pinion spacer then you end up using a crushable one that is set by the amount you torque the pinion nut. When under extreme torque load the flange/pinion itself can press inward further crushing the spacer and eventually increasing the backlash on the pinion to the ring gear. This can lead to premature gear failure.
Randy Rosetta KDØKWX TLCA #19351 69 FJ40 "Meanie" & 1970 Boss 302 Mustang