Totally depends on the situation. I have ran into homeless people who stink and assume they are that way due to mental illness (there is a place in town were homeless people can shower daily) and don't say anything....telling them that wouldn't help.
I have told family this (my kids) and feel like it is my job to gently tell someone who works for me this (though I have never had to do this but I have told people to wear less perfume when it is overpowering). I had to ask a mother to bath her child (I was a teacher for a while). I considered turning her into CPS (the child smelled like urine more than once) but worried that could have been the wrong way to go and just talked to her more than once and regularly talked to my school's counselor about it. She (the counselor) also kept in touch with this mom. We later found out that the mom was homeless....