Do I need proof of accommodation for my entire trip when arriving at BKK with a SETV?

When arriving in Bangkok (BKK) on a Single Entry Tourist Visa (SETV), it is advised to have proof of accommodation, at least for the initial nights of your stay. It is unclear whether you will need to provide proof for the entire duration of your trip, especially if accommodations are not yet booked beyond the first few nights. Furthermore, if you are connecting to Chiang Mai (CNX), you will go through immigration at BKK before your domestic flight.