Cedar does rot.
My dad built a new house in '77 here in central Indiana, and used cedar posts for the back porch, which were set into the ground.
Three years ago, he decided to enclose the back porch, and rebuild it completely. I was shocked to see these 8 x 8 posts extremely rotten....I dont know how or why the porch was still standing!!! They were so rotten that we had a hard time getting out the post part that was in the ground, because it would break off when we tried to pull it out.
Here are a couple of thoughts: Consider using steel posts, which are much cheaper. They are recyclable and will prevent new trees from being cut down. They also require less maintenance in paint, etc. Really want to spend your time painting fences?
While I am unsure of the damage creosote can create in the soil, you may consider using old telephone poles for fence posts. Sometimes they can be purchased from the utilities at low cost.
If you are set on new wood posts, you can add years of life by coating the end of them with boiled linseed oil. About $8 a gallon. Apply with a brush. It works just like paint. (Paint of yesteryear used linseed oil as a primary ingredient) This will help waterproof the base of the post. I would put 2 or 3 coats on if you have the time.
You can also 'paint' the tops of the posts with linseed oil, which will keep the rain from soaking into the post. BTW, it dries hard like paint.
Remember to used BOILED linseed oil, not raw.
clove