The NES leaderboards starts when you click Push Start on Round 1 and ends when it says Round Clear on the appropriate round.
We currently use the Game Select screen exit and the last block disappearing respectively. Do we see a benefit to syncing up these timing methods?
The Game Select Screen exit start point, I interpreted as the same start point "Push Start at Round 1." You start time at the same time you confirm the game options are correct to begin playing the game in both runs, and timing should start there in both runs.
There is about a half-second between time of last block disappearing in NES Tetris 2 and when Round Clear appears. For SNES, this difference is relatively negligible due to animation differences - Considering the times for the runs for this game on both consoles are currently to the second, I don't see any substantial difference in making SNES Tetris 2's endpoint "time ends at appearance of Round Clear" for run consistency. It's a much lower timing difference than making the NES one "time ends when last block disappears."
I don't see the few frames between last block and Round Clear appearing deciding SNES Tetris 2 records (yet), so if consistency among versions is important, changing the SNES run to "Timing ends at Round Clear appearing" seems best.