Depending on the "obstacle" sometimes you will grind on the chainring, but never really the BB. Most "freestyle" BMX bikes have huge rear drop-outs that form a large flange when butted up agaist a peg. A common problem in the past was for the edge of the "obstacle" you are grinding to pop over the flange and grind on the rear cog, or chain. To solve this problem, some of us now run our drivetrain on the opposite side that we grind on. Due to the opposite side drivetrain the new fad is to omit the pegs altogether and grind on the rear dropout/axel, and the crankarm/pedal (held in the horizontal position). As you can imagine this is pretty destructive to the bike but it gets you cool points at the skatepark to do "pegless" grinds.