I've cracked a bulkhead nut once before by overtightening it with a wrench so you do have to be careful not to over do it. I believe Tomanero is correct in hand tightening it as much as you can and then give it a good 1/4 to 1/2 turn with a wrench to snug it up.

I've never used silicone to seal them, but I have heard of other people putting a bead of it around a bulkhead lip before tightening it down.