Interactive Brokers does not have a special rule regarding stocks that trade below $5. Although other brokerage firms may have a house rule regarding stocks trading below $5, IB does not employ such a rule.
There are other IB-specific rules to consider in this scenario, however. In general, IB would not have a special rule for long positions in stock trading below $5, as long as they are exchange-listed. Once a stock is delisted from one of the exchanges and moved to the OTC market, it would be subject to a 100% margin requirement, since the stock would no longer be considered marginable.
Also for long positions, the maintenance margin requirement is 30% of the stock value or $2,000, whichever is greater.
For short positions, the maintenance requirement on stock greater than $5, is $5 per share or 30% of the stock value, whichever is greater. For short positions in stock where the last sale price/share is less than $5, then the maintenance margin requirement is $2.50 per share or 100% of the stock value, whichever is greater.