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 $55.65   13.131%   $62.9574   $62.96   $62.95   $63.00  
  Purchase #2 $136.67   8.767%   $148.6519   $148.65   $148.65   $148.70  
  Purchase #3 $22.59   10.272%   $24.9104   $24.91   $24.90   $24.90  
  Purchase #4 $107.44   8.682%   $116.7679   $116.77   $116.75   $116.80  
  Purchase #5 $180.87   4.841%   $189.6259   $189.63   $189.65   $189.60  
  Purchase #6 $67.96   3.136%   $70.0912   $70.09   $70.10   $70.10  
  Purchase #7 $52.62   5.181%   $55.3462   $55.35   $55.35   $55.30  
  7 purchase totals = $623.80       $668.3509   $668.36   $668.35   $668.40