Let's do some math ..  Rounding examples of 7 random hypothetical purchases:
  Case Tag  
Price  
Sales  
Tax %  
Raw Total  
Incl. Sales Tax  
Rounded  
to Penny  
Rounded  
to Nickel  
Rounded  
to Dime  
  Purchase #1 $24.58   12.829%   $27.7334   $27.73   $27.75   $27.70  
  Purchase #2 $88.42   7.853%   $95.3636   $95.36   $95.35   $95.40  
  Purchase #3 $76.76   11.760%   $85.787   $85.79   $85.80   $85.80  
  Purchase #4 $243.38   7.467%   $261.5532   $261.55   $261.55   $261.60  
  Purchase #5 $71.32   12.379%   $80.1487   $80.15   $80.15   $80.10  
  Purchase #6 $182.54   8.132%   $197.3842   $197.38   $197.40   $197.40  
  Purchase #7 $30.74   10.761%   $34.0479   $34.05   $34.05   $34.00  
  7 purchase totals = $717.74       $782.0180   $782.01   $782.05   $782.00