Do I need to provide proof of accommodation before my travel to Thailand if I arrive before my booked date?

I'm a U.S. citizen and will be traveling through Asia in June, and obtaining my visa in HCMC, Vietnam, at the end of June. I will tour Cambodia after that, then flying into Thailand. I already have a place to stay on July 5th and onwards, but it is very likely I will get to Thailand before July 5th, I'm just not sure of what date yet, however long it takes me to get through Cambodia. Do I need proof of where I will be staying on those dates before the 5th? Would I need this proof to obtain visa, or as I walk through customs at airport in Thailand? If this is the case, I will just pick a hostel at random and make a reservation. Correct? Thank you.