I usually keep the starter, and build the team around him tbh. Do you not find it depends which starter it is to how much you spoil them? I mean, in Kanto, I spoil my Charmanders because they need to be overlevelled to get past Brock and Misty, but if it's a Bulbasaur I'm using, he can usually walk through them, so I level my other pokemon more.
In Johto, I always level Totodile more, because I need Ice Fang to take care of Bugsy's Scyther (Rock Tomb is too inaccurate), but if it's Cyndaquil I chose, he gets levelled up in Sprout Tower and against Bug Catchers on Route 30/31, but then left as he is until Azalea's Gym itself.
Again, in Hoenn, I overlevel Torchic as it needs to evolve to take on Roxanne, but if it's Mudkip or Treecko, I give the others more attention.
In Sinnoh, as with Torchic, Chimchar gets overlevelled. However, so does Piplup so it's strong enough to stand up to Gardenia. Turtwig, however, doesn't get as much special treatment.
Overall, it generally depends how many pokemon I can find to cover their weaknesses, and if they are useful against the gyms or not.