Fence's don't have to be level, they have to be straight. There's a difference.
I use T-posts to pound in for my end posts on each property line, beyond the last post. Pound them in. Set the string between the two. It has to be VERY TIGHT. If i'm building a rail fence, where the height matters I also like to set the height too at this time. I set each string at the height I want the posts to be at using a tape measure. If you go more than about a 100 feet, you have to put posts in between because the string will sag.
If you are building a picket style fence, don't worry about the height of the posts until after you finish, then take a big old circular saw and whack the tops off. (much faster)
The trick is to get the post level in all directions and directly centered under the string. Sometimes it involves extra digging. Then, once it's set (and it takes two people to do this efficiently), I take scrap wood (actually those cheap 1x4x6' cedar pickets) and nail them in for 3 or 4 directions at an angle so it holds the post straight and level. This is the key. Then, your cementing person can follow behind without worrying about moving the post, or you either as you set the next posts.
does that help?
Oh, and being off side to side is much less noticeable on a rail fence than being off on height. It should gradually slope and follow the lines of the land.
HTH