When is it ok to interfere with someone else's parenting?

For example, where I live everyone puts covers on their strollers to act as a wind breaker. I generally only use my stroller cover for rainy days, and I’m always getting stopped on the street by concerned old ladies telling me I should get a cover for my poor child. I wrap him up well on windy days, and he’s at the age now where he only uses the stroller when he gets tired so it’s easier for him to hop in and out with no cover. It’s so humiliating being reprimanded in the street by complete strangers, and I can’t imagine doing this to someone.

Are there any circumstances where you would step in and give your opinion on a complete stranger’s childcare techniques? Have you ever, or do you think this is totally out of order?