I have renovated and changed dozens of sinks in my day. If there are no shutoffs under the sink you will need to shut the main valve off. Much of the time the main valve has not been operated in a long time and it likely to leak. If the main valve leaks and will not stop you will have a problem on your hands. The best approach is to have valves on all fixture supply lines. This way you will not need to touch the main valve. The cost of installing a couple of shut offs under your sink is minimal compared to replacing the main valve if it leaks. The other problem you may have is a leaking line near the sink. When this happens you will need to shut off the water supply to the whole home until the leak is repaired. This could be a major inconvenience.