I started some tubers between May 25th and June 11th back in 2011, and the ones I started later were actually later blooming large dinnerplate dahlias(85-90 days til bloom) and they started blooming in early September. I dont think its too late. In fact, they really dont start growing well until the temps get warmer anyway, which means they really should be planted when the daily highs are at least in the 70's to 80's.

If your dahlias are smaller, faster blooming types then maybe you will get blooms by mid August if you plant them now. Otherwise they will start in September. But thats OK because they will still bloom for nearly 2 months til first frost(depending on when your first frost occurs).

I am getting one last order of tubers this wkd and they are going right in, might be a bit late but we'll see! I also have pots that I started that are a few inches tall going in this week. I had to stagger plantings since I was also getting beds ready. Good luck!