It will depend on which type of stop you are using. If you use something like the 'double bid/ask' then there is the possibility that you stop will not fire if there are NOT two consecutive bids/asks at your stop price.
So to be safe, you would need to select the 'last' option which relies on just one price to trigger the stop.
The following is IB's wording of the types of stops:
quote
If "Last" is selected, stop orders will be triggered based on the last price. For example, for a sell order, there must be one last price that is less than or equal to the trigger price.
If "Double last" is selected, stop orders will be triggered based on two last prices. For example, for a sell order, there must be two consecutive lasts that are less than or equal to the trigger price to trigger the order.
If "Double bid/ask" is selected, stop orders will be triggered based on two bids or asks. For example, for a sell order, there must be two consecutive asks that are less than or equal to the trigger price to trigger the order.
If "Default" is selected, then the double bid/ask method will be used for orders for OTC stocks and US options. All others orders will use the last method.
unquote