Don't understand why you are worried about margin calls. They just transform your portfolio a little and you really don't lose much. They help to leverage your stake when the price is going up but if a turn is downwards direction about to happen I just adjust collateral to minimum get out bitUSD or bitCNY and let them be called. Whit this technique you earning more in uptrend than you lose in downtrend so you are overall in profit.
At the moment it is more profitable to trade against bitCNY because it grows against bitUSD. ( from 11. feb bitCNY goes up 3.6% against bitUSD).
Be good ;-)-
p.s. Reasions to use Bitshares: Why IRS can bust you on centralized exchanges and not on Bitshares! and my some favorite stuff @.