Laws about how long you have to actually be in the state to legally have a permanent address there
Ease of receiving and forwarding important mail, etc.
That last bit about the ease of receiving mail was a big enough factor to make me use my childhood home for my permanent address.
Granted, this address is only for snail mail and official documents from the state, financial institutions, tax documents, etc. and not like individual letters or regular packages from Amazon or other shops that I can have shipped directly to wherever I currently am at any given time.
How do you handle getting your mail while you're on the road long term?