91 octane gas, could it harm my engine? Options

[6strngs]

right, so normally, since my car is almost all stock, I just run it on the basic 87 which is the lowest they sell here. well, last time I went to the pump, I decided to fill up with 89, just to see what kind of difference it made. I feel like it added just a little bit to the accelleration, and I loved it! not only that, but I usually fill up when I've got a quarter tank left, which usually occurs after about 250 miles, but I've gone about 230 so far and it's just barely past the halfway mark... this could be a result of alot more freeway driving and alot less city driving though... anyway, I was thinking that next time I fill up in the next couple of days that I wanted to try 91.. but my friend said that it would cause my eninge to "ping". I asked if that was bad, he said no and that my knock sensor would pick it up and just flow more fuel in to remedy it, so I asked if it would give me worse gas milage that way and he said no it would give me better... can someone please explain this in possibly greater detail and tell me if there'll be any downsides to filling up with 91 or if it'll harm my engine or anything? thanks in advance!