JinKazaragi: In pokemon red I trained my starting team to lvl100 only by using the very first field of grass (I had way too much time back then.)
dtgreene: Be aware; if you evolve your starter Pokemon (requires leveling it up to level 18) before you get the Pokedex, the game becomes unwinnable, as Professor Oak will refuse to take the parcel.
(Of course, this isn't something the average player will do, but just be aware of that.)
This *may* have been fixed in some version, but the bug is still present in Yellow (but can't be triggered because there's no way to evolve that Pikachu).
Using a cheat to get a Poke ball and using it to catch a wild Pokemon this early will trigger the bug as well. (In Japanese Red/Green, using a certain glitch to sequence break could let you do it without cheats, but if you can do that, you can just glitch to the Hall of Fame to beat the game early.)
I did it after getting it so I didin't have any problem like that.