Standover height is an incidental measure of fit. What's important is top tube length which, based on what you've said, should be within a good fit range for you with the 57. It is true that some people like to size down on their frames, and that is largely due to current trends in the way professional cyclists fit their bikes. It is "fashionable" right now to have oodles of seatpost showing and steep saddle to bar drop, but that dynamic is not right for everyone.

edit- And I think a 50 or 53 would probably be much too small for you, but that is just one man's opinion.