When I first started riding, both me and the OH would check what gear we were each in at a certain point and tell if we found it easy or too hard. Then we realised that we aren't the same so what gear I'm in doesn't matter to him and vice versa. Rule of thumb we now have is ride what is comfortable for us. On a normal day I usually change gears whenever I feel it's either too easy to spin the pedals or too hard, but exactly what counts at "too easy/hard" changes almost every day.
Some days I want a challenge so grind a little, others I'm more relaxed so spin a bit. I don't get to a stretch of road and think "I need to be in 2-4 for this stretch" or "This is a 2-2 hill". Gear selection is a very "touchy-feely" aspect of riding for me, I tend to just know if it's right for that moment, and shift up or down if it's not.