I'm not sure it matters as long as the hole is big enough and propery prepared
my advice is not to plant it until its dormant, in a mild spell november to march and be sure its well watered at least for the first year in the ground

A square hole will make it hard to backfill the soil evenly around the the square root ball or round for that matter. You can scruff off the corners of the square root ball molding it to make it round, then pop it in.

makes little difference really. but as already said the hole needs preparing well, twice the size of the pot at least. enrich the soil you've taken out with compost/leaf-litter before back filling. if the roots are curled tightly around the root ball, tease them out a bit so they don't continue going round and round. I'd also plant in the autumn, making sure to plant it so it is at the same level as it is in the pot.

Well if the rootball's square, you'll have to dig a square hole (in a pre-prepared area which has all been dug over and enriched with composted material beforehand, not just a square hole in undug soil) to accommodate it. Othewise, agree with Seaburngirl.

Having grown up gardening in hard clay soil, I always make my planting holes square, and wide, since the roots get out better at the corners. I have had girdling roots form at the edges of round holes. I have dug a few star-shaped holes, but I am not really THAT O.C.D.! :)

I think you might be - just a little, Tug :o))

Wweell...notice that I didn't say that I wasn't O.C.D. at all! :D