calculate quantities at boundary grids in FVM method
 

hi everybody
I'm writing a code for solving diffusion, could I solve grids at faces at the time or its better that I wouldn't Solve those grids at boundaries and after solving whole domain,calculate quantities at domain? which is better and why? I am confusing to select a way for calculating temperature at boundaries for boundary condition type II
please someone say what is the advantages and disatvantages of each procedure.
thanks before