It's pretty common for BMX sprockets to be out of round because a lot of 'em are either cheaply made or they're made by small companies that don't have the means to machine them accurately. Turn your bike sideways. Turn your cranks all the way around a couple turns. Does the chain fall off? If no, then you're good, you can stop stressing. Your chain should *not* be super tight, only enough tension to keep from falling off using the above test. Tight chains wear cogs and sprockets and stretch out a LOT faster. Worn drivetrains = dropped chains, chain skip, horrendous crashes.