I think there is some significance to it, but like anything it's not 100%. Think more in terms of seasonality for specific stocks. For example, check out MWV, OMX, ODP, and SPLS over the years in the July-September timeframe.
Since 1998? That's called "small sample bias". September and October have generally been the worst months for the market based on long-term data. The biggest and best meltdowns have occurred during those months. The "Sell in May and go away" adage is being replaced with "Sell in May, stay sober until October, so you can buy bargains that you can sell in May at a profit". You heard it here! Stay sober just a little longer.