Optimal pH of protein buffer? Basic principles to adjust buffers according method and analysis
The correct pH depends on the properties of the proteins that you are trying to exploit. These are a good deal different between extracting proteins from tissue and performing Western blots.

Extracting proteins means keeping them soluble in aqueous solution. Working at the pI of the protein might be the worst choice as the protein then has no net charge, typically reducing its water solubility. An extreme case would be the highly basic histone proteins with very high pI values but best extracted at low pH.

After proteins have been run on a denaturing gel and transferred to a membrane for Western blotting, they are already denatured and bound strongly to the membrane through hydrophobic interactions little affected by pH except perhaps at extremes. For the blotting, you want to exploit the interaction of epitopes on the protein with the antibodies against them. During antibody production, the interactions between the epitopes and the antibody-specific receptors on the antibody-producing cells occur at normal extracellular body pH, 7.4. Using that pH for the blotting best recapitulates the charge state of both the epitopes and of the antibodies as they existed when cells producing the best antibodies were selected.