Are there rules about using the "Fasten seatbelt" sign outside of take-off/landing/turbulence?
You dont have a jurisdiction tagged (although it sounds like this is an EASA matter) your answer may vary depending on where you are asking for. I can answer for the US/FAA. 

Here in the states you are technically in breach of an FAA FAR by not remaining seated. As per FAA FAR 121.317


  Sec. 121.317
  
  Passenger information requirements, smoking prohibitions, and additional seat belt requirements.
  
  ....
  
  (f) Each passenger required by Sec. 121.311(b) to occupy a seat or
  berth shall fasten his or her safety belt about him or her and keep it
  fastened while the "Fasten Seat Belt" sign is lighted.
  
  .....
  
  (k) Each passenger shall comply with instructions given him or her by a crewmember regarding compliance with paragraphs (f), (g), (h), and (l) of this section.


I cant find a regulation on using it in times its not otherwise required by FAA FAR 121.311 but this discussion seems to elude to the fact its at the "Captains Discretion".