It depends on the maturity of the cedar and even if it is Eastern red cedar, there's differences depending where you live. The Eastern cedar we have here is not good for posts...we get too much rainfall and the heart just doesn't get a lot of red in in it and it doesn't get hard. Go farther West up where Fenceman lives, and it's completely different and even tho some of it's the same species.
I cut some 4"-6" cedar here in 2010, let them dry a couple months and used them on my yard fence for line posts. Most are now rotted out except maybe 1-1 1/2" of heart. Same with the juniper. We have it here as well as the Eastern, and neither of the local stuff make good posts. Out in San Angelo or around Johnston City and they use lots and lots of ashe juniper, which are really good, but very little of it is cut much East of I-35. The higher rainfall in this part of the state makes a lot of difference in how good the tree is going to be for a post.