wowowowo, you dont have to evolve it, but if not, the game will be very challenging, and when it's challenging its kinda fun, I beat it all with charmander, no regrets here :D proud to be a ? Nonevoler...
It'll still learn some nessary moves...
But you have to train hard and often, also your other pokemon got to be over par, and ready for anything...
You dont have to evolve it, just do what you want :D Just swing it and have fun!
I personally would love not evolving it, as you can go around boasting that you beat the game with a bulbasuar, \o/