No. Whoever told you that should stfu. Season 5 is good. Season 7 is good. 6 and 8 have their flaws but you still get some good Dexter moments.  8 is a rough season but one of the worst parts was the ending, and since the ending has now been expanded on and the story continues, it takes away some of the sting of it. Plus you need to know what happens to understand New Blood and Resurrection.