I have some experience growing dahlia from seeds but not from tubers. Picked some up and want to plant them up in containers to see if they will sprout. Should I water them in after planting? I've read conflicting things about that. Some say it encourages rot.

I do not water until I see sprouts.

I try not to water until the sprouts are above ground (for garden planted tubers). If it's super hot with no rain, then I do water lightly. I grow about 25-30 in pots and water those everyday if it hasn't rained, regardless if they've poked above soil or not.

Oh thanks, Melissa! Now I know what to do with the potted tubers. I initially sprinkled the soil with water to dampen it and after I planted the tuber{in gallon pots} I ran the kitchen sink hose over the soil. Not enough to soak them or have the water come out of the bottom of the pot. They're on the table getting north exposure. Should I keep these watered until I plant them?

They usually sprout fairly quickly if they're somewhere warm and sunny.

If it didn't work out this time, maybe start smaller unless you have a super large tuber.

I have used Ziploc bags with a small bit of a corner cut off and the zipper cuff turned over to good effect. They don't take up much space, they don't take much potting mix, they don't take much water and they're super easy to figure out when they need more water because you can see the roots and you can see the mix is truly dry.

I lost very, very few tubers starting them early this way.

No water until you see the green growing up thru the soil line.

What if they already have leaves on them before you plant them? How deep are they to be planted?

You can plant them a couple of inches deep and then continue to mound up as they grow. I do that when I start tubers early.
You still don't want to water a lot unless you know they've got a good for system, though.
They can sprout and grow from a tuber for a while without roots, only using the nutrients from the tuber. That's sometimes why sprouted plants wilt and die for no reason. No roots to maintain the growth after a while