Car guards I will tip if and when I deem it necessary, like if he was friendly and helpful and willing to take the trolley away for me and what not. I have no problem presenting him with a crisp tenner if he deserved it. He's making an honest living and I respect that.
If however I'm not even gone for 2 minutes, didn't bring any trolley, or if I'm in a position where I haven't got a cent in cash on me, I simply give the guy a thumbs up and say thank you very much. They know me and they know I'll be back again where they might get another tip. There's one who even knows my name and knows the cars I bring, and jumps up at the sight of my car as soon as I pull in, frantically indicating to me that there's a decent parking spot waiting if there's one available. This is of course at my local shopping centre that I frequent almost daily.
At malls I park underground only, and if a car washer approaches I simply smile and tell them no thank you and they leave it be. Have never experienced aggression with these guys. I dunno, maybe it's because I'm bigger than all of them. Maybe it's because I'm simply friendly towards them.
Now, window washers at the traffic lights are another story. Same with beggars. I do not and will not ever support them, and I used to. Some are aggressive, and I had an incident once with a window washer in Pretoria CBD which ended up with me aiming my ex-5er at him and gunning it. He jumped; I missed. End of story. Beggars I cannot afford to support. I pass by around 12 beggars one way each day. So 24 in total on return trips to work and home. These guys would have more than I do if I had to support them all every day (slight exaggeration intended), and besides that they're actually capable of getting a job if they tried, even if it is gardening or packing boxes in a factory because not one of them have a disability that's visible to me.