Well-drained soil is key for dahlia. Mine are in raised beds, with the majority on the southern face where it's very warm.

Don't water the tubers when you plant, the soil is damp enough unless it's bone dry. Dahlia won't generally require any watering at all until they're six inches tall or so.

Planting before frost ends isn't necessarily a no-no, but dahlia dislike sprouting until the soil temperature reaches 60. As long as the plant hasn't sprouted and the soil doesn't freeze, they won't be harmed by frost (but it'll slow them down).

Here in eastern PA, my target date is usually between May 1 and May 10th--which is a touch early as there's still some chance of frost here at that time (but a small chance). I do it early because my annuals have to go starting May 10th-May 15th, and that's a lot of work. The dahlia don't mind.