I know we can use 'em as short form of them in informal English:
short form of them
Tell 'em to go away. Cambridge Dictionary
but the dictionaries don't mention when we can/cannot use them.
in the following example the author use 'em and in the same pargraph he use them.
Them shoe heels was so high they couldn't hardly stand up, much less walk in 'em. They must a-been three inches high, and them heels tapered down real keen at the bottom. Gratitude for Shoes
So, When shoud we use 'em and when shouldn't we?