We use second person plural in order to show kindness or respect, but sometimes in order to show social distance or lack of intimacy. It always depends on the circumstances.
It is very polite to use plural form for people that you don't know well, especially if you are in a big city, where people generally speak more formally.
In small cities and villages there is less formalism in both language and lifestyle. People use second plural less often. Sometimes they avoid it not because they are impolite, but in order to make you feel familiar and comfortable, as a part of their hospitality.
In any case, either you are in a big city or in a small village, it is absolutely acceptable and polite to use plural when you talk to people that you don't know well (especially if you are not a native Greek speaker).