So the definitive answer is this.
IB has the ability to front run their customers. Part of it is the right you signed away on your margin account so that they can lend your shares and better offset risk.
I forgot where I found the research. But it was on HFT when one of them caused a flash crash with the first recorded use of spoofing.
The research went into tick by tick replay of how it happened and discussed what kind of structure allows a brokerage to front run customers and spoof as well.
If I remember correctly, Charles schwab is one of the ones that cannot do this. In general, any brokerage that allows smart routing is basically front running you. Smart routing is basically asking for it.
But in a sense. Since there are so many HFt doing front running and spoofing now, I believe the benefit for them fron these activities are close to zero. The most they might get is a cent or two, which for most retail investors who's trading at the fastest is in the 5 second interval, is negligible. Especially so if you buy and hold more than a day.
Disclaimer: all anecdotal evidence of course. I tend not to save links to researches because there are too many and just incorporate it to my knowledge. If anyone wants to rebut me by askibg for proof, I have none and I am not in the business of proving my theory so that others can make money competing against me.