If you add amendments to the soil, the roots may not want to venture out of the original hole. You want the roots to spread out and go in search of nutrients. Finally, before you tamp down the dirt, make sure the bush or tree is straight. Trust me, you plant one crooked tree and no one will ever let you forget it. The compost will add nutrients slowly to the soil so your plant can use them as needed.
Inorganic fertilizers can lead to excessive growth.
The last thing you need is a fall plant that starts putting out fresh, delicate new growth. The new growth makes the plant much more susceptible to damage when the cold weather arrives. Too much mulch will prevent water and air from reaching your plant. Don't let the mulch touch the trunk of the tree - it can damage the bark and lead to disease. Finally, water well.