From the Soldier and Cop side: I had "P" ask me what kind of knife I carry or carried in Iraq. Well, I own about 100 knives, but the one I seem to be the happiest with and I carry all the time is my EMERSON CQ7B folder. I bought it when they first came out in the 90's and prefer it over all others. I have carried on patrol as a cop, I always had it in my jean pocket in Bosnia on missions and took it everywhere I went in Iraq. I still carry it today. I know if I have to use it for some nasty business, it won't break or fail me.
On police patrol I also carry a newer knife made by Gerber. It's a rescue type that has a screw driver kit with it and comes in really handy for normal everyday cop crap. Again, it's not a toy and will hold up to anything I need it to do and has a cool little seat belt cutter thing on it.
Knife rules: Keep them sharp and clean, don't play with them, treat them like your life depends on them to work and be there when you need them.
Did I carry a big sheath knife in Iraq? Nope, I had my Emerson folder and I knew it would be right where I put it and didn't have to worry about loosing it or cutting myself with a bigass knife.