I have a 29qbs and originally had one sway bar. My first few tows I noticed a few times where trucks gave me some sway (not a lot but enough to give me pause). The sway bar manufacturer (Curt) indicated based on my length that I needed a second sway bar.
Whether I had just become better at loading, WDH use, etc. (I was fairly new to this at the time) or whether it was the sway bar, it seemed to help quite a bit. For the price, it certainly was worth trying and I am very glad I did.
I also upgraded my shocks to Bilstein and added Timbrens to my TV, again, well worth the investment.