Well, while it was certainly a huge can of gasoline thrown onto a raging inferno, I'm not sure this alone could delay the ACW too much if at all.
Simply put, by the time Dred Scott came about (1857) Bleeding Kansas had already been going (John Brown had been involved for two years), Brooks had already attacked Sumner...in other words the "war" over slavery was already happening.
The biggest possible butterfly is that OTL it spurred Lincoln's "House Divided" speach, so maybe Lincoln never gets the Republican nom in 1860. Maybe without the controversy in how this contradicted Northern Democrat's "popular sovereignty" banner Douglas and company aren't as discredited...and so maybe there isn't such a massive split in the Dems leading to the OTL three-party run...
...soooo, I guess we could see Fremont (or whoever repalces Lincoln) defeated by a (somewhat) unified Democratic ticket...possibly delaying the inevitable a few years. Biggest short-term butterflies will IMO be specific to individual politicians' careers.