Definitely get the permit if it makes you feel more secure.

Take this with a grain of salt, but you really don't need to worry about cops too much in Portland pulling you over. My bf drove with an expired temporary license plate for 3 months after buying a car(just forgot tbh) and there are stolen chop shop cars driving around Portland with no bumpers or license plates at all. I've seen trucks driving down the street with "I don't think that's legal" wooden self made tiny houses built into the back. You should be good, and not worry too much once you get your temps.