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 $11.72   12.221%   $13.1523   $13.15   $13.15   $13.20  
  Purchase #2 $113.94   9.852%   $125.1654   $125.17   $125.15   $125.20  
  Purchase #3 $21.87   6.718%   $23.3392   $23.34   $23.35   $23.30  
  Purchase #4 $207.83   4.867%   $217.9451   $217.95   $217.95   $217.90  
  Purchase #5 $233.42   12.819%   $263.3421   $263.34   $263.35   $263.30  
  Purchase #6 $11.90   12.165%   $13.3476   $13.35   $13.35   $13.30  
  Purchase #7 $64.27   6.432%   $68.4038   $68.40   $68.40   $68.40  
  7 purchase totals = $664.95       $724.6955   $724.70   $724.70   $724.60