I am not sure Mavishka is right in saying you could work for anybody or start a business after getting a GC through NIW, unless of course the new job or business has some relevance to the NI claim that one made in their petition. The US is admitting a person under NIW under the assumption that this person is going to be making contributions to the US economy in that field, right? So if a researcher doing cutting edge drug development starts a restaurant business a month after his GC approval, it does beat the whole point of NIW!
Although there is no written directive on this issue, to be on the safe side it might be better to have a job or business at least somewhat related to this field at least for a few months or a year after approval. Otherwise like someone else said, it will look suspicious. You never know the ways of the UCIS, they might decide to initiate a few random checks on NIW GC holders! It might become a issue during Naturalization, even though that's 5 years down the line, better to be safe than sorry!
silent