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 $12.79   4.518%   $13.3679   $13.37   $13.35   $13.40  
  Purchase #2 $30.27   7.250%   $32.4646   $32.46   $32.45   $32.50  
  Purchase #3 $64.45   3.319%   $66.5891   $66.59   $66.60   $66.60  
  Purchase #4 $129.81   8.617%   $140.9957   $141.00   $141.00   $141.00  
  Purchase #5 $7.70   6.174%   $8.1754   $8.18   $8.20   $8.20  
  Purchase #6 $112.70   3.386%   $116.516   $116.52   $116.50   $116.50  
  Purchase #7 $231.60   9.266%   $253.0601   $253.06   $253.05   $253.10  
  7 purchase totals = $589.32       $631.1688   $631.18   $631.15   $631.30