Can you try converting your baq field to a string value in a calculated field and see if that helps the situation? I know excel can get stupid when copying dates, numbers, basically anything that can be formatted as something else.
SSRS could be another beast, it might not at all be related to your BAQ. Within SSRS there is way to bake code that runs at run time, add expression to fields, etc.
Can you post the expression for one of the fields that you are having issues with? Can you also explain what the rounding error is with an example? Are you expecting 9.99999999999999999 but getting 9.99 instead as an example.