This kind of risk always exist. Even if you use a super-reliable broker, better be ready for unexpected. A backup is necessary for any serious trader. Of course, 24 is better then 23.5 but it should not be a killer.
I would not recommend any rules of thumb. CBOT and Eurex government bond futures are settled in delivery, for example. Lean Hog on CME settle in cash. IPE oil is settled both ways. It is absolutely necessary to know what you are trading. IB does not accept delivery option to reduce cost of...
IB will liquidate your positions before you have a chance to make or receive delivery http://www.interactivebrokers.com/en/trading/marginRequirements/physicalDeliveryLiquidationRules.php?ib_entity=llc
Take a look at Interactive Brokers Canada IDEALPRO. We are not making markets.
http://www.interactivebrokers.com/en/trading/forex.php?ib_entity=ca
You can find a plenty of related discussions on elitetrader. For example, http://www.elitetrader.com/vb/showthread.php?s=&threadid=60032