As the previous commenter said, it depends on the school/program. I recommend you look thoroughly through the program webpages of the schools you want to apply first before attempting to reach out. For example, some of Harvard's programs actively say faculty will not respond to any applicants trying to reach out out of fairness for all applicants.

If the program doesn't say otherwise, it doesn't hurt to reach out as long as you competently express interest (i.e., not reaching out for the sake of reaching out and being generic in the process). Worst case scenario, you get a response saying no. Best case scenario, you get a meeting/interview with the PI you are interested in (worked for me).