From what I've heard there have been "some" instances of the pennies wearing through the rubber slightly (possibly people adding too many) resulting with minor damage to the paintwork where it sits.
Either way lot's of people have done the penny trick successfully by using pennies, nylon washers, rubber washers etc so it's up to the individual to decide what they want to put into the boot popper to get the same result, ...a one button press boot opening.
Edit: spelling.
I've got 5 rubber washers in each of my poppers but with the weight of my nismo v2 wing it still fails to pop with one press!