Hi Gail,
It depends on how you are getting your sales figures. If you are using the LY / YTD figures in the AR Customer record, then yes, these sales will be lost. If you are getting your sales figures from either OrderEntry and/or SalesAnalysis then you'll be OK.
Why not take a copy of your data to your local drive, run a sales report, then delete the customers and run the sales report again to see whether the number has changed?
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Andrew Bates