If you are considering a joint JD/Tax LLM, consider the University of Washington. If you work on them concurrently, you get to count 12 of the 36 LLM units towards your JD (double dipping rocks). And, you don't pay the LLM tuition rates on the units you use towards your JD (so you save 1/3 on your LLM).
You still have to make it to 36 units in the LLM, but you shave 12 units (effectively) off your JD.
I just finished my JD and am working on wrapping up the LLM next year. Personally I love tax classes, so it has worked out really well for me. The firm I am at this summer is also very excited about getting a new tax LLM too.
How does the University of Washington place outside of Seattle? I wouldn't mind going to Washington state, but I'm concerned that I wouldn't be marketable outside of the state of Washington. Where is the firm you are heading to located?