Yea, be very careful. This is why so many 70 cudas at the shows have the grille trim not holding very well.
I have heard that you can take a hot iron and re-mushroom the tabs if one gets loose on you though. That is if it doesn't completely brake off. It can be done without breaking it though as dutch mentioned. I restored my grille and this is how it turned out.
I restored my trim as well and it is now installed even though it's not on in the pictures.
Good luck!