I tend to agree. I've had a few blends that I hated from the first couple of puffs -- largely due to their high nicotine content. That kind of thing isn't going to change down the road. . . . and I'm not going to try; I simply cannot smoke high-nic blends.
That said, I try not to review/rate blends until I've had at least 10 bowls. After all, it takes time to burn away the previous blends. And some blends take time to figure out in terms of ideal moisture level, packing approach, and smoking speed. Never mind that often our perceptions are colored by the time of day when we smoke, what we last ate, the weather, our overall mood. Lots and lots of variable that can affect what we think of a given blend on any given day.
Assuming you don't hate a blend early on or are made sick by it, I'd definitely recommend folks stick with a blend for at least half a tin (which, for me, tends to be around 10 bowls).
Bob