Fridge hack for breastpump

Hi mummies, anyone using the fridge hack after pumping for your newborn? Is it safe ? And usually after how many pumps do you wash ? I haven’t deliver my firstborn yet but thinking of doing this for in between late night pumpings. Will the residue from the previous milk go bad inside the pump? Since we wont be washing it and will continue to the next pumping session. As i heard some say its not safe. #FTM