Production Design
Objective-ish…
Rain Man certainly isn't filmed from any one character's "perspective"—which is why we used the word "objective." We see Charlie from all angles and in interactions with lots of different people, and we also get to follow Ray around in his private moments (and when he wanders away from Charlie).
But We Did Add "Ish" …
However, we added an "ish" because the camera is more grafted to Charlie than any other character. Charlie is the first dude that we meet, and the last one we see—and it's his personal journey toward non-crummy behavior that the film largely follows.
This is his story, and the fact that the film begins and ends with the camera focused squarely on his experiences definitely attests to that "focus" (pardon the filmy pun—sometimes we can't help ourselves).