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 $236.97   12.312%   $266.1457   $266.15   $266.15   $266.10  
  Purchase #2 $213.41   6.807%   $227.9368   $227.94   $227.95   $227.90  
  Purchase #3 $26.29   12.674%   $29.622   $29.62   $29.60   $29.60  
  Purchase #4 $36.50   12.572%   $41.0888   $41.09   $41.10   $41.10  
  Purchase #5 $132.52   10.864%   $146.917   $146.92   $146.90   $146.90  
  Purchase #6 $169.59   7.702%   $182.6518   $182.65   $182.65   $182.70  
  Purchase #7 $92.40   3.240%   $95.3938   $95.39   $95.40   $95.40  
  7 purchase totals = $907.68       $989.7559   $989.76   $989.75   $989.70