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 $52.59   12.206%   $59.0091   $59.01   $59.00   $59.00  
  Purchase #2 $217.37   9.462%   $237.9375   $237.94   $237.95   $237.90  
  Purchase #3 $247.68   6.507%   $263.7965   $263.80   $263.80   $263.80  
  Purchase #4 $113.83   3.266%   $117.5477   $117.55   $117.55   $117.50  
  Purchase #5 $105.27   6.308%   $111.9104   $111.91   $111.90   $111.90  
  Purchase #6 $67.94   9.186%   $74.181   $74.18   $74.20   $74.20  
  Purchase #7 $115.32   6.237%   $122.5125   $122.51   $122.50   $122.50  
  7 purchase totals = $920.00       $986.8947   $986.90   $986.90   $986.80