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 $128.78   11.341%   $143.3849   $143.38   $143.40   $143.40  
  Purchase #2 $66.57   11.274%   $74.0751   $74.08   $74.10   $74.10  
  Purchase #3 $9.62   8.384%   $10.4265   $10.43   $10.45   $10.40  
  Purchase #4 $110.75   13.714%   $125.9383   $125.94   $125.95   $125.90  
  Purchase #5 $13.77   3.273%   $14.2207   $14.22   $14.20   $14.20  
  Purchase #6 $148.31   6.527%   $157.9902   $157.99   $158.00   $158.00  
  Purchase #7 $131.70   5.274%   $138.6459   $138.65   $138.65   $138.60  
  7 purchase totals = $609.50       $664.6816   $664.69   $664.75   $664.60