There's not really much of a rule, except that if you make a change that you KNOW is really big or controversial and mark it as minor, you're going to tick off the NPOV umpires.
The only other thing, which is I think why there's a check box for "minor edit" in the first place, is that a lot of people don't want to see minor edits: the "view minor edits" option is off by default.
If you've merely corrected the spelling of they're to their, or fixed a [[broken link] to a [[good link]] -- I might not want to know about it. (Except for the anti-vandalism squad, who keep "view minor edits" on all the time).
Ed Poor