Well, if you're not going to be using it in battle, it doesn't really matter when you evolve it - so as soon as possible.
However, if you do decide to change your mind (because Raichu is awesome) then you'll need to look at what moves Pikachu can only learn when it hasn't evolved. If you keep it as a Pikachu until level 29, it learns the fantastic, standard Electric-type attack Thunderbolt. It has high power, high accuracy, and gets a STAB boost when used by Pikachu/Raichu.
I'd evolve it at level 30 or so. Most of the moves it learns after that point are pretty useless, because Thunder/Discharge are worse versions of Thunderbolt, Feint sucks, and he's too frail to set up Agility/Light Screen. Most of Raichu's best moves can be taught to him via TM.
Hope I helped :]