Don't know about IB for options but even if it was just for regular stock how wouldn't that be increasing your buying power? Also there's no real such thing or point in holding cash in reserve for a margin call you simply have a loan ratio you cannot go above. Instead of holding cash you just transfer the cash in to your margin loan accounts and it "pays off" the loan or reduces the balance of the loan which will decrease your ratio - no difference between holding cash and putting it in your margin account except with holding cash it will actually make you get the margin call in the first place.
