I almost always park in parking garages. Much better peace of mind, in my opinion. I've never been anywhere in the city where a parking garage wasn't easily located, at least within a block or two of my destination. In the provincia, I'm usually going to a store or a friend's house, where there's parking. After a couple of incidents of parking on the street and having my car towed where I could see no sign prohibiting parking, and other cars parked there too, I decided for as little as I drive my car, a nice berth in a garage does wonders for peace of mind.
A buddy of mine has an apartment near Recoleta Shopping and he used to have to deal with a guy there who insisted on asking my buddy or I for money every time we would park in front of my buddy's apartment (he has a car too). My buddy used to get quite incensed that someone was insisting that he pay for the privilege to park on his own street. After a few rounds of this, the guy gave up trying to collect from my buddy, but I always slipped him a 10 pesos bill when I was parking because he would find me a space on the street (the only place I can remember parking on the street where one of these guys was in business) every time. He had an old beater that he parked in a space and if nothing else was available he'd move it for me to park there. He actually provided a service, wasn't there for extortion.
The reason I mentioned that - quite coincidentally, the guy who used to control parking on my buddy's street now works for the garage where I park my car (couple of blocks from the apartment). Heh. We've become friendly and he always does a great job on my car when I get it washed - 130 pesos inside and out, detailed, etc. I just wish he'd quit asking me if I can set him up with one of my sisters-in-law, who he's seen a number of times when we all go out somewhere
The windshield washers though...used to be they were real easy to deal with. Simple wag of the index finger and they'd leave. Seems like in the last half year or so they've become more aggressive, at least around Micro Centro/9 de Julio/Libertador. Tell them no and sometimes they start putting dirty soapy water on the windshield anyway. That really pisses me off. I start becoming rather emphatic and turn on the windshield wipers even if they have a mess they should clean up.
It was one of the few things I liked about Buenos Aires more than Asuncion. In Asuncion they are very aggressive and I've had more than one incident where I had to leave the car to prevent them from breaking the windshield washers when I told them "no". It used to be pretty nice in BA - if I needed the windshield cleaned I'd pay to have it done, but when my windshield is clean I don't want the service. Nowadays I get a hard time from about a third of the guys here in BA.
I don't mind paying for a service I want or need - but I don't like to be forced into it. In fact, I often give the guys who do juggling and other stuff at the lights if they're good and I enjoyed their "show". but I don't give money to people just because they insist!