Of course you should announce. In my park system, it is required to announce any passing. Of course the iPod users can't hear you, but most do. I also usually get a wave and a "thanks" when I pass someone.

This idea of "if there is enough room I don't announce" is just wrong. You are startling people for no reason other than you don't want to be polite. Yes, I do stay to the right and expect others to do also. I will even say something to people that don't. But I also use this path when I walk my dog to the dog park and do expect others to pass me. I am staying right but a biker zipping pass me with no warning is startling.

Depending on how fast I am going and who I am about to pass determines how far away I will announce. Typically, this means about 4 or 5 seconds before I get to them. The path I ride the most goes through the woods with many blind turns. I do not pass anyone in these situations. I will tail the biker in front of me until I am at a spot where I can see it is safe to pass, then even if they know I am there, I will announce my intentions. This really helps with families with small children out riding.

One time, I was on a MUP and saw a lady sitting on a bench and a dog on the other side of the path. Something didn't look right to me, so I announce my intentions to pass a sitting person. Luckily I did because she had that dog on one of those long skinny leashes. I would have rode right into that leash if I did not announce.

Oh, and I say "Passing Left" seems to work most of the time, but some do get confused. But with my 4 to 5 second warning, I have plenty of time to make an adjustment.