I just went through this yesterday. I bought a 3 month tbill and was going to leave a fair amount of cash to cover drawdowns but I was told that even if I did use up my available cash they would automatically sell the tbill in $5k increments with a $10 transaction charge. No biggie if it were to happen.
So then the question is - do you keep your futures account fully funded to account for use of T-bills or simply keep enough to cover your trading and put the cash elsewhere...