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 $27.65   8.472%   $29.9925   $29.99   $30.00   $30.00  
  Purchase #2 $228.40   8.455%   $247.7112   $247.71   $247.70   $247.70  
  Purchase #3 $223.72   3.412%   $231.3533   $231.35   $231.35   $231.40  
  Purchase #4 $62.25   5.129%   $65.4428   $65.44   $65.45   $65.40  
  Purchase #5 $76.31   4.338%   $79.6203   $79.62   $79.60   $79.60  
  Purchase #6 $102.80   3.709%   $106.6129   $106.61   $106.60   $106.60  
  Purchase #7 $139.41   12.790%   $157.2405   $157.24   $157.25   $157.20  
  7 purchase totals = $860.54       $917.9735   $917.96   $917.95   $917.90