Plotting grid only apply to the display of the results. Since Pro/MECHANICA uses polynomial function for the solution of displacement, strain and stresses, this functions are continues from node to node. Therefore, you could find results at any point between the nodes. What the plotting grid does is to divide the element boundaries and faces in subdivisions at which results are plotted (output). One may deduct that if you increase the plotting grid you may have more points in the grid and therefore a better fidelity for the solution. However, while that statement is true you need to considered that what MECHANICA outputs at the end is not the raw data but the results of superconverged stress values which is a way to smooth the stress across the boundaries of the element. The superconverged stress results is a method of improving stress accuracy and estimating stress error. Superconverged is used by the single pass adaptive algorithm to increase the polynomial when it tries to converge to a solution by comparing the raw data to the superconverged solution. At the end only the superconverged solution is output. Which personally I don't completely like since looking at discontinuities in the stress and strain field usually will tell you how good the solution is. If MECHANICA will out put the stress error in the element will serve the same purpose but at this point that functionality is not available.
Consequently, increasing the plot grid may not have much impact in your over all solution and will only increase the analysis output files. My recommendation is to leave it alone at grid plotting of 4 which is the default. You could always change it but I doubt the results will differ by much unless the element is in the vicinity or part of a singularity.