I've had NH weigh my bags twice out of the last 15 trips. Once was ex-US and another was at NRT. Both times I was using a carryon that appears bulky (22" Kirkland old style). And both times, while exceeding whatever limit there was for carry-on luggage, was permitted to proceed because I told them I was carrying a medical device (CPAP) in it.

Since switching to a slimmer 20" rollaboard in 2014, no one has asked. And it has been perhaps 6+ trips already with the new bag since last year.

So I think it helps if your main carryon luggage has a lower profile. And I have never ever had them ask for the "personal item" (which is a computer bag or backpack) to be weighed, even when my rollaboard was.