Where do you live in the US? In the UK I'd say now so long as the ground isn't frozen.

I would agree with Mg, but add that if your weather is anything like ours, make sure it's not flooded either! Derek.

Ah well there is that Derekm

Autumn/winter is ideal for planting hardy trees, however choose a period when it is frost free, dig a decent size hole and add some decent/compost leafmould and stake/tie properly, as regards shrubs yes some hardy types will be ok, but we are past the best time for planting these, early fall would have been ok, the best time would be early spring when the soil is warming up and its getting a little milder, plants are not cheap nowdays so why risk it also you will have a far better selction of shrubs at the garden centre than you have now through the winter, give them a good start with some compost and give them a good mulch.

In my area, most trees & shrubs prefer to be planted in fall or winter, so their roots get established before the heat of summer. Frost tender tropicals are less of a risk planted in the spring, and palms are best planted in the summer, since their roots grow best in very warm soil.
In the coldest parts of the States, spring, shortly after the soil thaws, is the best option, so the plants get fully established before facing the possibility of frost heaving. Some bulbs and perennials can be planted in the fall there, though.