Having moved around 3 different countries in the past 6 years, putting the country code before a phone number is basically second nature to me at this point!
Never. It's a relic of the dialling system that is no longer necessary here in the US unless (I guess) you're dialling internationally. I can count on one hand the number of times I've dialled internationally in the past decade so it just adds visual clutter and I remove the +1 any time it pops up automatically.
As it's pretty common to have international phone numbers in the contacts over in good old Europe, I always put the Swiss country code (+41) in front of all my numbers. I would go crazy seeing some numbers with, and some without country code 😂