The way I play, experience groups never really affect me that much. I just rotate Pokémon when one of mine levels up, so that none of them really do fall behind.
...But actually looking up those experience groups, now I see why I felt like I was underleveled in
Gold and
Silver. Umbreon and Alakazam are the only Pokémon on my
Silver that aren't in the Slow group (yes, I actually rotated Typhlosion out to make room for Houndoom), although I may replace Skarmory with Scizor once Dragonair evolves and I can replace its Waterfall with Fly.
My
Gold team levels up quite a bit faster in comparison: three Medium Slows (Meganium, Poliwrath and Nidoqueen), two Medium Fasts (Jynx and Steelix) and only Dragonite as a Slow. I'm at the same point in both games, but just about everyone is 1½ levels higher in
Gold.

But really, that's just something I never really stop to think about. It really only comes up when I'm rotating a new Pokémon into the roster and it needs to level up a lot to evolve, as with Tynamo or Larvesta in
White. Other than that, I don't really take it into account and just level-grind as I normally would.