Having Gen 1 Yellow, the hardest part is when realizing that after Erika(whose Ace is level 32), the Ace of the next Gym Leader in Koga is level 50(which is a huge jump). Same deal with Sabrina's Psychic Pokemon who are also level 50. Blaine's Ace is level 54 and by the time you get to him, you should have Pokemon near 50, if not above it if you grind while Giovanni is level 55.
Considering the strongest E4 Pokemon is Lance's Dragonite at level 62, you can pretty much get away with having your Pokemon around 57-60. However, I typically try and grind my way to 65 to match the Ace of the Champion(which is usually a level 65 Jolteon in Yellow). While I may be on equal terms level wise, I purposely don't evolve my Pokemon so as to have lower HP and stats(making it just a teeny bit harder to win).