Do you mean the value that gets passed from a checkbox on a form field? That will be "on" or "off". So you can do the Subscribe conditionally if the field is "on", or Unsubscribe conditionally if the field is "off".
We have found in our set up with Eloqua and SFDC, the value can be: true, 1, on, Yes (with an uppercase, not lower case)
It's a pain building out filters when you have to take this all into consideration. I'm not sure how this has happened but there it is.
Yes, this can be a big pain. Best bet is to just pick one and stick with it and make everyone aware.
I thought a checkbox on a form, before it is saved to a contact field, always returned "on" or "off"? Not until it is saved to a contact field will it then translate into whatever you set the values to be for checked and unchecked (which you probably want uniform, yes).