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 $17.70   3.499%   $18.3193   $18.32   $18.30   $18.30  
  Purchase #2 $91.59   12.612%   $103.1413   $103.14   $103.15   $103.10  
  Purchase #3 $140.34   4.150%   $146.1641   $146.16   $146.15   $146.20  
  Purchase #4 $60.41   12.111%   $67.7263   $67.73   $67.75   $67.70  
  Purchase #5 $44.91   7.510%   $48.2827   $48.28   $48.30   $48.30  
  Purchase #6 $18.39   7.656%   $19.7979   $19.80   $19.80   $19.80  
  Purchase #7 $85.67   11.683%   $95.6788   $95.68   $95.70   $95.70  
  7 purchase totals = $459.01       $499.1104   $499.11   $499.15   $499.10