I am posting this, because I could not find the answer online, and I ran the test to find out. I appreciate that it is a better practice to define every scale that will be used, but I was curious which direction Revit would default, should I not.
If you have line weight 4 set to 0.0160" at 1/4" scale and 0.0120" at 1/8" scale, but no settings for 3/16" scale, the 1/4" setting (0.0160") will apply to the line weights at 3/16" scale.
I hope this helps someone.