The good thing about ROTH is that it doesn't follow the IRA rule of 72T. You never have to withdraw anything and you can give the whole thing to your heir. By using their life expectancy for withdrawal, they will be able to receive a large sum every year tax free...
Ocean, you said that you would like to do a one time transfer of your TSP to a ROTH IRA. Now, this is where the tax will come in. Griffin and ColonialMike had some input with this topic. I recommend for you to get a life insurance that will cover the tax that you will get hit for the transfer. Here is a scenario: Let's say an individual has a million dollars in his TSP when he dies. His beneficiary is his wife. The wife inherits the TSP but must start withdrawing and close the account within 5 years or do a one time withdrawal or a transfer to a ROTH. If she picks the latter, she can transfer the 1 million dollars into ROTH, pay the $360k in taxes (36%) using the insurance money. By using this method the ROTH IRA you plan to leave behind for your heir starts off in 1 million vs. $640k. By using the compounding interest for a rate of return, and his wife lives for another 10 years, the difference will be substantial and his heir will never have to pay a cent in taxes...;-) See excel worksheet below.