A lot depends on White's play!
After 1.e4 e6 2.d4 d5 3.e5 c5 4.c3 Nc6 5.Nf3, Black has options:
5...Nh6 - This is a fairly recent idea. It gets decent coverage in "The Even More Flexible French".
5...Bd7 - I personally don't like this line at all for Black. After 6.Be2 f6 7.O-O fxe5 8.Nxe5 Nxe5 9.dxe5 Qc7, White seems to be better after both 10.f4 (the positional approach) or 10.c4 (the aggressive approach). Notice the Knight here is still undeveloped! Some older French books cover this line heavily, like the French Advance, 2nd Edition, by Sam Collins.
5...Qb6 and now:
6.a3 Nh6, headed for f5 after 7.b4 cxd4 8.cxd4 Nf5. Black can also play the old main line, 6...c4, which after 7.Nbd2 Na5 (Forced), the g8-Knight usually goes thru e7 and c8 to b6, going for a Queenside attack while White attacks the Kingside. I prefer 6...Nh6, personally. Both lines can be found in The Even More Flexible French.
6.Be2 cxd4 7.cxd4 Nge7 8.Nc3 Nf5 9.Na4 Bb4+ 10.Bd2 Qa5 11.Bc3 b5 and Black's slightly better. I always play 6.a3 when I'm White, never 6.Be2, against the 5...Qb6 line.
So notice that there is no cut and dry answer to where the King's Knight goes. It all depends!
I really enjoy playing the French defense, I find it very solid and pleasant as Black. However, I always find myself unable to settle my king's knight (the one that starts on the g file) on a good square. I usually play Ne7 followed by Ng6 or Nf5. Which of the two is the better option? I am talking of the advance variation of the French defense, which is what I almost always have to play against.