Having come to this game from a Warhammer background, I've always been super-conscious of the legalities of pre-measuring ranges and/or arcs.
For those unfamiliar with Warhammer, up until a few editions ago, pre-measuring of any kind was explicitly forbidden by the rules. Eventually the rule was flipped on it's head, and pre-measuring became explicitly legal. It was a big change to the way the game plays, but was fairly well-received by most players.
As far as I understand the rulebook and FAQ for X-Wing, pre-measuring is completely illegal. You can't check range until you have firmly declared a game action that requires the range to be checked. Yet I still see it happening all the time, even during competitive events, often by players who I know are serious dudes who play a lot, and have tourney or even regional wins under their belts. I even noticed it happening semi-regularly during the Worlds on the live streams of games.
So is this a rule people are just unfamiliar with, or are players making a conscious decision to ignore the rule during casual play (which might then slip into their competitive outings)?