I've been asked a few times to remove the plates. If the owner ever seems nervous or upset about me taking photos, I tell them right away that I can shop out the plate, and they're usually fine with that.
This photo is from the last time I saw my neighbor's Gallardo. Someone had spotted his car the night before and posted it to the Bay Area Car Spotters facebook group, and he was pretty weirded out (in a laughing kind of way) when I told him about it. He let me take all the shots I wanted, and even cleaned up the car so it wouldn't be dirty in the shots, but he asked me to blank out the plate before posting it online. I thought that was kinda weird, since 1) he has a vanity plate, 2) he has plenty of distinguishable modifications on his car, and 3) it had been posted online plenty times before. I still blanked it out though; I never want to upset a supercar owner.
Flickr link