It depends on whether the non-final forms learn a move or three that the final form doesn't, and on the limits of my patience. For example, I like Seed Bomb on my Bulbasaur, but that means I have to wait until L40 to evolve him, because Ivysaur and Venusaur won't get it and there's no Heart Scaling that.
On the other hand, Trapinch learns Earthquake and Fissure at very late levels, but practically speaking, I'm not willing to wait on his evolution that long. Also Fissure is extremely inaccurate, so I TM or breed Earthquake onto Vibrava/Flygon. STAB is STAB.