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 $59.98   5.650%   $63.3689   $63.37   $63.35   $63.40  
  Purchase #2 $27.74   10.253%   $30.5842   $30.58   $30.60   $30.60  
  Purchase #3 $113.95   3.873%   $118.3633   $118.36   $118.35   $118.40  
  Purchase #4 $171.59   6.581%   $182.8823   $182.88   $182.90   $182.90  
  Purchase #5 $59.85   12.720%   $67.4629   $67.46   $67.45   $67.50  
  Purchase #6 $117.73   6.143%   $124.9622   $124.96   $124.95   $125.00  
  Purchase #7 $70.72   8.742%   $76.9023   $76.90   $76.90   $76.90  
  7 purchase totals = $621.56       $664.5261   $664.51   $664.50   $664.70