Well, the rule says "No", you shouldn't add stickers or markings to the robot or strategic objects.
Rule 4 -- EQUIPMENT
-- EVERYTHING - Everything you use in the competition area directly or indirectly for strategy (mission-related activity) must be made entirely of LEGO-manufactured elements in original factory condition.
- Stickers are not allowed, except LEGO stickers, applied per LEGO instructions.
- Paint, tape, glue, oil, zip-ties, etc. are not allowed.
— Exception 3: Marker may be used only in hidden areas, for ownership identification.
[Personally, I've even given up teasing teams about using marker on their robot, because they get upset. And it's not practical in many situations anyway.]
Your referee may tell the team that the stickers are not allowed, by Rule 4, and that they should take them off. The bottom line is that the team gets at least three chances to show their highest score, so that an accidental disconnection or misconnection in one match should not be the end of the world. Knowing the robot, and designing it for to make bad connections unlikely is part of the overall challenge for the team.
VA/DC FLL Referee Advisor