Ten years is a long time to trust your eye alone. What's the largest format tile you regularly set without any layout lines? I've seen guys get away with it on small subway tile in a straight run, but anything over twelve inches, especially on a big floor, can drift. That slight error multiplies across a room.
Learned this the hard way after a bathroom floor went sideways. Used to think my eye was good enough for sixteen inch tiles, no lines. Then I read a post from @luna891 about drift and it clicked. Now I won't touch anything over eight inches without a chalk line. It just isn't worth the fix later.
My last kitchen backsplash used twelve inch tiles with no chalk lines. The wall was only eight feet wide so any drift stayed minimal. Maybe the fear is overblown for smaller, contained areas like that.