To be honest, it doesn't matter too much as dahlia tubers produce shoots all over the tuber. Examine the tuber carefully and see if you can identify the buds which will form the new shoots.
Place the tubers shallowly in moist compost and give them lots of bottom heat, around 70F. You can then take the well grown shoots and use them as cuttings to make lots of dahlia plants.
Alternatively, just plant the tubers reasonably deeply, about six inches down, in late spring, and the tubers will grow into single dahlia plants.