I think one thing you can assume going off everyone's widely varying opinions on which season they think is best, is that the "quality dip" is largely based on the opinion of the person, and not a general consensus like it was with heroes. 
Now, I will say this, while I think Season 1 the best season, even my least favorite season (season 3) is still better then most other tv shows out there. When I rank my least favorite Dexter shows, I'm ranking them in comparison with the past Dexter Seasons, not TV shows in general.  Dexter, thus far, has not experienced any kind of quality drop remotely similar to Heroes. His weakest seasons are still quite engrossing.