Quote from Hoi:
Provided that he is a pure DayTrader (no holdings overnight), and trading 95% futures and seldom stocks or FX.
If you trade 95% futures then you should keep your cash in the CFTC sub-account since:
(a) your funds from futures trading would not be eligible for SIPC coverage even though they are swept to the securities sub-account.
(b) not keeping the cash in the CFTC sub-account would mean that you are giving up preferential treatment of your claim in a bankruptcy.
Check out:
http://www.nfa.futures.org/nfamanual/NFAManual.aspx?RuleID=9059&Section=9
Specifically, the FCM should disclose that by transferring excess funds from an FCM's customer regulated commodity accounts, the customer will not receive the preferential treatment afforded funds held in a customer regulated commodity account pursuant to Part 190 of the CFTC's Regulations and the U.S. Bankruptcy Code.