I have a new home with rough-in plumbing in basement and I was wondering if I need a vent down there. I do plan on putting in bathroom
if you mean a vent in the plumbing, then yes. wet or dry, the plumbing must be vented. if you mean an air exchange vent to pull moist or smelly air out of the room, then it may or may not be code in your area, but do yourself a favour and just put one in. its always a good idea.
if you have open plumbing in there, i would buy rubber caps to seal the pipes until you install the bathroom to keep sewer gases from being able to enter your home. If you are talking about a vent for the plumbing itself to help it drain then you can buy an air admitence valve that will allow air into the plumbing pipes but will not allow the gases to escape to help with drainage. This will save you from having to run a separate vent pipe for the basement