Zone 6 sounds a little chilly for leaving them in the ground over winter. Are you in the mountains or in Eastern Oregon? Here I have pretty good luck with deep planting and then mulching over the winter, but I usually loose a few each year.

On the other hand, with only 1 dahlia, its not that big of a deal to dig it if your soil is in good condition. Its fun to see how well it has reproduced under ground, and I like to pre-start my dahlias so I get more bloom time. Especially this year the ground has been so slow to warm up. I am just planting my potted dahlias out now, but some are 18" tall. I try to get my tuber down 6" to withstand our winter freezes.

The joy of having all that color in August into Nov makes it all worth while for me!