I have very similar questions, but first the background...So, the questions I ask myself when moving something to/from my SM list are "is this something I want to move on this week?" and "is it urgent/important enough to commit to doing something with it this week?" If the answer to one (or both) of these questions is "no", then it goes on the SM list.
When I was first "coached" in methodology like this, I had an inventory of 180 projects. Yup, 180 "things" that I'd said I was going to do (and, I was a high school teacher!). Within a year, the someday/maybe list grew in to the hundreds.
To this day, I still have outstanding projects, areas of focus, and someday/maybes... and weekly I ask myself:
"IS there an action I could take this week to get any more experience in this area?"
"IS this something I'm still even interested in?"
"HAVE I reviewed this enough times in my weekly review to now let it go?"
I'm one of those guys who can open the newspaper and find 4 more people I'd like to write a letter to. So, having a way to manage the "have to," "could do," and "might do" lists is crucial to my health, and quality of life balance.