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 $227.66   12.569%   $256.2746   $256.27   $256.25   $256.30  
  Purchase #2 $180.61   13.456%   $204.9129   $204.91   $204.90   $204.90  
  Purchase #3 $99.41   3.214%   $102.605   $102.61   $102.60   $102.60  
  Purchase #4 $236.50   9.191%   $258.2367   $258.24   $258.25   $258.20  
  Purchase #5 $200.75   4.583%   $209.9504   $209.95   $209.95   $210.00  
  Purchase #6 $217.53   7.874%   $234.6583   $234.66   $234.65   $234.70  
  Purchase #7 $165.43   4.337%   $172.6047   $172.60   $172.60   $172.60  
  7 purchase totals = $1,327.89       $1,439.2426   $1,439.24   $1,439.20   $1,439.30