There's a bit of legality involved as well. If you were hired in location X and then go work from location Y, the company has to pay you according to location Y's laws. (Very layman's explanation, that's not the entire story). So it's definitely best if you just get permission to do so. That said, there are some managers/companies who are a bit more flexible on it. Whether or not they should be is another question, but I've had some coworkers in the past working internationally for a bit and everyone just kept it under the table.