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 $11.99   4.575%   $12.5385   $12.54   $12.55   $12.50  
  Purchase #2 $121.43   3.109%   $125.2053   $125.21   $125.20   $125.20  
  Purchase #3 $24.79   10.281%   $27.3387   $27.34   $27.35   $27.30  
  Purchase #4 $121.41   6.497%   $129.298   $129.30   $129.30   $129.30  
  Purchase #5 $65.64   3.328%   $67.8245   $67.82   $67.80   $67.80  
  Purchase #6 $96.68   3.364%   $99.9323   $99.93   $99.95   $99.90  
  Purchase #7 $165.44   7.589%   $177.9952   $178.00   $178.00   $178.00  
  7 purchase totals = $607.38       $640.1325   $640.14   $640.15   $640.00