should timing be set "by the book", or tweaked?

[cell]

My Haynes manual indicates my idle advance should be 4 degrees BTDC. Is that it? End of story? Or is this something which should be fine tuned?

I have heard horror stories of people shortening the life of their motors by "tweaking" their timing, and my 292 doesn't have any provision for knock sensing, so I don't see how I would even know if I had pushed things too far until my pistons start grenading...