That entirely depends on your setup and expectations. The vast, vast majority of customers can bolt the neck on and go. If you are looking for an exceptionally low action or unique feel, some work may be necessary.
Like Wyliee says, most people are good to go, but my SS frets came with pretty sharp edges. So most, not all. You shouldn't order with the EXPECTATION that it will be perfect upon arrival; always budget in a proper setup. Worst case scenario, the neck arrives, doesn't need work, and you have an extra couple hundred bucks in your bank account.
On my first two builds, I was pretty satisfied with the fretwork "out of the box." I then got a neck with a dead spot. I found a local luthier who did AMAZING fretwork on this guitar. Since then, I took the previous two builds to him, and every one since. Warmoth does state on their website that you should consider a fret dressing, as they do not dress the frets. Trust me, it's worth it. Great action to be had, as well as smooth fret edges. Plan on spending about $130 for a fret dress and setup.
Yeah, mine needed the ends dressed as well as a leveling. I've yet to do the leveling though. Fortunately I like fairly high action, and I'm getting buzzing but it's not coming through the amp. In 1 or 2 spots a little overtone comes through, but pretty okay otherwise. I am about to spend a bunch on some proper leveling tools though and I look forward to eliminating all buzzing as I have on my other electrics. (I figure I could pay the local guy $70 and get an okay job, or spend twice that on tools and have everything I need to do it right now and forever.)
The nut was fine for me though, although I'll see if it needs lowering after I level the frets. Out of the box, the nut was better than any factory Fender or Gibson job I've seen (although maybe a high-priced custom model would have a better nut). Some people who prefer lower height will want to go ahead and lower it though.
Well, saving for something and expecting to pay for it, but then not needing to seems crappy to me. It was a wasted effort, but at least you ended up with some extra money. It might mean waiting for a few extra weeks/months, so I guess I value time more than money.