This is a tough one. I didn't amend the soil for my cherry trees at first and they drowned in our wet clay soil. Dead trees don't grow out and adapt.
You don't want a pot of perfect soil while the rest of your yard is crappy soil. Like Drew said, they'll girdle or refuse to go out.
What I do is to make the soil half way between the best soil for that plant and what I've got around it. Mostly it's about drainage, not increased nutrition. I don't make it too rich with fertilizer like he said. Mostly I'll add gravel to help drainage, some old wood for improved drainage and fungal soil. I"ll make sure I plant it high. Then over time, yes, adding mulch or some other organic material so the worms, microbiology and other life can get that soil active and create nutrition.