In my opinion, it is highly unlikely that Penson will go under in a way that would cause customers to lose their deposits. But just in case I keep the majority of my account in the FDIC insured securities side of TDA/thinkorswim. Per MFG if you guess that a 30% haircut or so is about as bad as it will be, then only keep 40% or less in your Penson futures account, so that way your worst case should be losing 12% of that trading account.
If you are smart and have more than one trading account, and only have a reasonable portion of your liquid net worth in all your trading accounts, then that 12% loss in one account will not amount to all that much overall.
If you are smart and have more than one trading account, and only have a reasonable portion of your liquid net worth in all your trading accounts, then that 12% loss in one account will not amount to all that much overall.