I'm really "all of the above." I really just play it by ear at different times, with different situations and people. Also, depends on what type of mood I'm in too. Sometimes I'm feeling very talkative, then other times I'm rather quiet. I've had people be rude to me no matter which way I went with it, either announcing or not. One time, I passed an elderly man without announcing and got absolutely scolded for not letting him know I was coming.
The very next person I encountered, an overweight jogger about a mile down the trail, rudely told me "you don't own the damn trail" when I politely said on your left ...having just learned my lesson from the elderly man. You just can't please all the people, all the time.
For the most part, if someone "is holding their line" not showing any erratic behavior, I don't find it necessary to announce. Especially someone wearing earbuds that probably won't hear me anyway. If it's a group taking up the entire trail, I'll sound out a demanding "coming up to pass on your left folks" so they have time to organize. With kids, I almost always have to give very detailed directions, being super cautious and slowing way down. Coming up at full speed and just announcing on your left and doing nothing more with them, is asking for disaster. I always compliment them on a great job as I go by. Makes their day.
On a more humorous note, one time I was approaching two very attractive female runners, and was gonna announce something like "coming up fast behind you, ladies." Somehow, it came out as "coming hard on your back, ladies!" It drew quite a hardy burst of laughter and giggles as I rode by.
I find that on busy weekends when everyone has their brother and mother with them, I simply just get tired of announcing all the time and eventually get to the point of not giving a rat's azz. I'm still courteous and cautious, especially with the kids, but for the most part I just shut up, ride and don't worry about it.