Is it legal to put money into other people's parkiing meters?

I've been doing it since I was a kid; it was one of my late dad's hobbies. But recently, someone told me it has become illegal and cops now write tickets for this. So...do I need to watch for cops when I do it?

@greenmountaingal: they'll probably write you a ticket

@Goralski: but do it anyway!

@Peaches: but of course:)