Thank you for your replay.

I understood that if I did not specify "--surfreg my_avg_subject.sphere.reg"  mri_preproc was going to use the original file (sphere.reg), because in the help of mri_preproc it mentions this: 

--surfreg SurfReg : default is sphere.reg

With this in mind,  do you think it would not  be necessary to specify it in mris_preproc?

What is the difference between sphere.reg and  my_avg_subject.sphere.reg?

Best wishes,