Policy varies among FCMs (Futures Commission Merchants -- the futures equivalent of Stock Brokerage Houses) and in some cases, among different brokers within the same FCM. Man Futures, for example, has lots of different policies at its various arms, because Man has grown by acquisition and the FCMs it acquired had different T-bill policies which they don't dare change for fear of alienating their customers.
Many will let you use free cash to buy a T-bill and then allow 95% of the face (maturity) value of the T-bill to be used towards margin. If you buy a 13 week T-bill with face amount $100K and yield of 5.00%, it costs you $98,765 plus approx $45 commission to the FCM. 13 weeks later it magically turns into $100,000 and they don't charge commission when T-bills mature. In between, your $98.8K investment in a T-bill, can be used as $95K towards margin on futures positions. ($95K is 95% of the $100K face amount).
However, if your (cash + Open Trade Equity) dips negative, the FCM will "break" (liquidate) your T-bill in order to get your balance positive. They will also charge you approx $45 commission to do this. So most people leave a bit of extra "cushion" or "safety margin" in the form of extra cash that isn't in T-bills (and isn't earning interest). Who benefits from this? The FCM, of course.
If you shop around, and if you have a sufficiently large account, you can find FCMs who don't monkey around with T-bills at all. Instead they just pay you a money market interest rate on your daily (cash + open trade equity) balance. It's usually pegged to some index, such as "LIBOR minus 50 basis points" or "EONIA minus 75 bips" etc. The advantage of this is, you don't have to mess around buying and selling T-bills every week, and it pays interest on ALL your cash; there is no "cushion" that earns nothing. The disadvantage is, if the FCM goes Refco (bankrupt) then more of your money is potentially at risk.
Finally, there are FCMs which also operate stock brokerages (example: Morgan Stanley). If you are a big enough player you can negotiate a deal to let assets in your stock account, serve as margin for your futures account. You can buy T-bills etc in your stock account, and let them margin (at some reduced rate like 95% or 90%) your futures trades.