The real question is where you are going. I'll start with a simple answer on the voltages then come back to this.
When designing a circuit, the inputs to the processor must allow the voltage. In addition, the circuit is designed to take a certain amount of power from the power circuit. The rest of the circuit for a consumer product is generally designed to be durable and contains things such as clamping diodes to prevent current surges and over-voltages.
The simple answer to this is therefore we can not know exactly unless we have the specs for the parts or a general datasheet was released, which I did not look for. It is safe to assume Nintendo has some really good circuit designers however, and the device therefore probably has at least a 10% tolerance from what you usually charge at. I wouldn't worry so much about stuff like this, as you can tell pretty fast by whether the power regulation circuit on the device side is heating up. Worry more about the country standard.
Your true concern is that when traveling overseas, the AC mains voltage changes. Here in the US it is 110, and likewise, our products are made to deal with that. In Japan it is 100, so if a product is designed for Japanese market (note that the DS has an American compatible version which is different than the Japanese one), then it will have problems over here. If you took an American product there, then it won't charge as fast. Then we have the European standard at 220V. If you plug in your device over there without a power converter, say goodbye to your device. Be weary of this and if you have to buy an adapter anyway (which I suggest), then you may want to just wait until you are there and get a brand spanking new charger.