Here's another reason I think if you're going to have a policy then the signs would be a good idea . . . a lot of times the FA's are not going to enforce or even announce the policy. For example, my flight the night before when they invited us to move around
The only ones (other than JetBlue) affected by "self-upgrading" are the other passengers on that row (there were none on my flight). Having signs makes it possible for those passengers who paid for extra legroom and a good possibility of an emptier row to address someone who, at the start of a long flight, comes and sits down in the empty middle seat of their EML row. That strikes me as a perfectly legitimate objection, and as I said I'd not have moved into the seats if any of the row had been occupied.
If you put up a sign, it lets the passengers who did pay to sit there point to the sign and say, "Hey, just so you know before you settle in, they'll be by in a few minutes to collect the $xx fee for the upgraded seat". As someone who does buy EML seats on cross-country flights, I'd see that as valuable.