They are usually pretty cool but know if they see you twice w/o #'s they won't be so cool. Be ready for an inspection when you do get pulled over.
I'm 2 for 2. Multnomah Co. got me once and Clark Co. got me once. I've been waiting on some special order #'s and I showed them the reg and they were cool. But did they did make a point to say If they say me again they wouldn't be so nice. Oh and my new #'s are now on.