carpenters. "Measure twice and cut once." I think this can also apply to watches...before buying and before selling. I am down to just three Rolexes. One I love, one I like a lot, and one I like but am trying to sell (even though it is the one I most recently purchased and maybe the rarest of the three).
However I just took it off the market because I've decided it makes a great beater for those days/moments when I don't want to use the other two (gardening, jogging, hiking).
The way I judge a "keeper" is by how much I wear it. Other collectors may have other methods of rating their own watches. But for me, if it isn't on my wrist I quickly lose interest in it. This was the case with some mint versions of rare models that I had years ago but that I was always worrying about when I wore them...and also with some watches that I bought because the mood struck me and I had the money, only to put them away and lose interest in them almost as soon as I bought them. So now I make it easy on myself...if I'm not wearing it, I have no interest in owning it.