Like in every Fallout game, Big Guns (Yes I know that's not a skill in NV) are a waste of time and money. By the time you get something like a the Fat Man or Minigun and enough ammo, you'll already have an infinite supply of ammo for your Combat Shotgun, Sniper Rifle, or Assault Rifle. Wait a bit more and you will have Energy Weapons that are on par or exceed the power of Big Guns.
The only game to break this rule is FO2 purely because of the Bozar. The game was deliberately designed so some skills aren't completely useless, and Big Guns got a good buff with the Bozar.