When Series 14 hit, I was smooshing at Legoland California one day. One of the cashiers saw me doing it, brought out a new box, told me to buy it, smoosh at home, and return whatever I didn't need.
They actually encouraged me to open up bags if I was worried, and that I could return those to, but I told them that was sketchy and unfair. I don't think they care, because they use a lot of the current CMFs for the minifigure trading. So they open up bags all the time.
Other times, I've had shops direct me to certain cashiers (at locations around the park) who are known for their smooshing skills. I know everyone laughs at the "3 second smooshing" technique, but I've actually seen Legoland employees that can do it with amazing accuracy.