Most people lay the tuber sideways when they plant them. The sprout will find it's way up. You want to avoid watering tubers until the sprout is about 3" tall otherwise you'll either cause it to sit in damp soil and rot or in the case of a couple I grew my first year they swelled and split and then rotted. I'd water it once it gets warm outside enough to keep the soil from being bone dry.

Here is a link that might be useful: Swan Island Dahlias growing tips