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 $13.74   12.168%   $15.4119   $15.41   $15.40   $15.40  
  Purchase #2 $10.32   9.650%   $11.3159   $11.32   $11.30   $11.30  
  Purchase #3 $13.70   4.638%   $14.3354   $14.34   $14.35   $14.30  
  Purchase #4 $139.80   10.695%   $154.7516   $154.75   $154.75   $154.80  
  Purchase #5 $56.39   4.109%   $58.7071   $58.71   $58.70   $58.70  
  Purchase #6 $93.33   4.282%   $97.3264   $97.33   $97.35   $97.30  
  Purchase #7 $148.34   6.325%   $157.7225   $157.72   $157.70   $157.70  
  7 purchase totals = $475.62       $509.5708   $509.58   $509.55   $509.50