Technically you do have to re-register a TM30 after 24 hours in a different province. But this is Thailand and it seems that every immigration office enforces it differently. I'd assume that they're all requiring it unless you're explicitly told otherwise by an immigration officer at your local office.
I've often booked a "pay later" hotel and cancelled the booking later. There are also fully refundable air fares (though they tend to be bloody expensive!).