Now there's a question that's bothered me for a very long time: When is the best time to plant? In the end, I'm not sure there is a right answer.
To me, the best time to plant is when you're going to take care of your plant. It sounds rather silly and somewhat simplistic, but it really is true. Fall isn't a bad time to plant and it can be a really safe time to plant trees, but too many people don't take care of their plants properly.
Planting early in the year like spring or summer gives a plant time for it's roots to take hold. Spring is great because you have the moisture in the ground and it's not too hot so there's less maintenance. Summertime planting needs more care to keep the plant moist but most people accept that and water the plant properly. In the fall, however, plants get dropped in the ground and forgotten.
If you are going to take the plunge and plant a tree, or shrub, or anything in the fall that needs to live the winter, give it proper care. In the fall, it's more important than ever to baby along those roots. While many people forget the bone meal and plant start fertilizer other times of the year (still not good), it should never be forgotten in the fall. Whatever you can do to give those roots a hand in the fall should be done in order to have success.
Another thing to watch is water. Many plants cannot take dry soil when the frost comes. It's important to make sure the plant has adequate water. This is more difficult to do with a freshly planted tree because the hole will have a different amount of moisture than surrounding soils. If the plant has too much water, ice forms in chucks and starves the roots. Too little and the roots get too cold and die off. There needs to be some moisture to form crystals to protect the roots but still allow air to get to the roots. Experiments freezing plant roots in blocks of ice vs. moist soil have proven this. Some trees, like red oaks, are very sensitive to moisture in the fall so be diligent.
Planting in the fall isn't a bad idea. It's easy on the plants to move with little to no root shock or risk of wind-burn from being moved. Just make sure to do your part and take care of the plant with bone meal, plant start, and proper moisture. Ultimately, the best time to plant if when you do actually plant and provide the necessary care.