Plan sounds good. All kinds of good advice from Bob. I wouldn't worry about the forming tube, but that's me.
Wait to stain. If you want to paint, there is a way to get it done early, but I can't remember the procedure. I can find out if you'd like though. You can also use a clear sealer which is popular too. Let's you keep that nice fresh wood color.
Another tip if you're working alone, and even if you're not. Stretch two strings, one high and one low. If you can get these two perfectly plumb all along then you can set the edge of your posts along these and you'll stay inline (colinear is the term?) and plumb in one direction. They also make a nifty corner mounted level that you slap on there and can optionally pin it in place and check two directions at once. (If you use a high line, make sure you can detach one end or do something to prevent clothes lining yourself and prevent yourself from tripping.
Have you bought or rented a post hole digger yet?
Sure of your property line? Do you need a survey? Would hate for you to build a nice fence and then find out its 6in on your neighbors lot.
Enjoy
Jason