The project is coming to end and go live on the new system is just around the corner. The standard operating procedures have been written, flow charts created to illustrate the new business processes, development carried out to support where you felt there were gaps in the standard system, all configuration is done, master data transferred, users trained.
In other words, you are ready to push the big button and take your staff from a very well known (although outdated) environment into a new and different way of doing things. Different screens, different way of working, and perhaps even a new role because the introduction of a new system with other capabilities often dictates organizational change.
The answer is YES only if a very detailed integration testing has happened. The last thing you want to deal with when you are running on the new system is small bugs, questions about how exactly to handle a particular business process or how to run a particular report. The best way of creating a test to validate that you can perform your business and all of your business on a new system is integration testing.
There is no way around it. Do it before go live and you have increased your chances of success, do it after — and you will face a lot of unhappy users and you are off to a bad start...