When most folks speak of correct development times, it's usually in reference to proper tones on grade 2 (for fixed grade paper) or #2 filter (for VC paper), but I wouldn't say it's a "cast in stone" standard. Many photographers I know have settled on a 2.5 filter as their "standard." Back in the days when I used only graded papers, I tailored my MF / LF negatives for grade 2 and my 35mm for grade 3. It really all boils down to the look you prefer in your prints.