Well, they do seem to do that. Most people were shocked at an old Cylon like Tigh, while I was not. I was in a small minority however, who hopefully read the clues correctly.

However, the general dramatic rule for big surprises in a mystery or suspense story is you're supposed to go "Aha!" when they reveal it, and suddenly see how it all fits. "The Sixth Sense" is one of the great examples of this.

But it isn't always followed. Sometimes Rosebud is just a sled. And in this case, they only picked the 4 Cylons mid-season so there were no "aha" clues specifically for anybody but Tyrol, but there were certainly clues that it could be anybody, even people like Tigh who had fought in the first Cylon War.

I hope they will stick to the dramatic principle. You may not be able to figure it out for sure from the clues they give, there will always be some uncertainty, but when we learn, it should be both dramatic and make sense.