As I was planting the dahlia's on friday I did come across the odd one left out to over winter in the border that hasn't put up any shoots, some won't and some will come later, on the whole though they don't seem to have taken much harm and that was with temperatures down to minus five at times. I think the biggest threat to dahlia's being left out is not so much the frost, although that will kill them, but the wet which can cause the tubers to rot. If you have a sheltered border that doesn't get water logged then I wouldn't bother to dig up and store. One thing you do have to watch out for if leaving out is slugs that eat off the new shoots as they emerge so leaving you thinking that dahlia's didn't come up. This time I'm going to over plant the dahlia beds with wallflowers which should offer an additional layer of protection through the winter.