@hansamann, the Photon keeps a FIFO list of up to five WiFi configurations. When the Photon connects, it will go through its list in a first to last order to attempt a connection. There is no mechanism currently to “tell” which WiFi to connect to.
So, by setting credentials at home then at other locations, the Photon will take a little longer to connect but will work its way through its list and connect.
Don’t forget that adding more than five configurations erases the oldest one (FIFO). You can erase all configurations by holding down the SETUP button for more than 10 seconds if you wish.
If you search the forum, there are others who have asked similar questions and some have use the Credentials API functions to clear and set WiFi credentials but there is some concern regarding wear on the memory (eeprom possibly) used by the Broadcom/WiFi chip on the Photon if excessive writes are done.