It depends on where you are planning to go and how many people you will come into contact with. Our pediatrician wanted us to wait three weeks with our daughter (full-term) and two months with our son (preemie & in NICU for a few weeks), to allow their immune systems to develop sufficiently to be able to handle most of the germs they'll come into contact with.
So for a stroll around the neighborhood, with no interactions with other people anticipated, you could go out sooner. For a visit to a family reunion, wait a while.
Anecdata: we had to take our daughter at 3 days to get formula (poor planning on our part!) so we wrapped her up so she was basically a bundle, and zoomed in and out of the store. No interactions with people wanting to touch and kiss and hold the newbie.