I think at the end of the day the simple answer is this - only give a charitable donation if you know 100%, beyond a doubt, that the recipient will appreciate it / supports that cause. You can't even make assumptions based on "Oh Millie's father had cancer, therefore she'll support X cancer charity" etc, because as we all know, that simply isn't necessarily the case - charities are a very contentious issue as people have their own thoughts about the contribution, use of funds, and politics of various organisations. If a person genuinely wants charitable donations and has expressed a charity of preference, it's not rude. But it's certainly not a gift that should be given willy-nilly or without thought.