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 $9.80   12.335%   $11.0088   $11.01   $11.00   $11.00  
  Purchase #2 $246.98   9.753%   $271.068   $271.07   $271.05   $271.10  
  Purchase #3 $131.56   10.228%   $145.016   $145.02   $145.00   $145.00  
  Purchase #4 $18.59   4.266%   $19.383   $19.38   $19.40   $19.40  
  Purchase #5 $12.60   7.723%   $13.5731   $13.57   $13.55   $13.60  
  Purchase #6 $113.94   7.103%   $122.0332   $122.03   $122.05   $122.00  
  Purchase #7 $76.45   9.696%   $83.8626   $83.86   $83.85   $83.90  
  7 purchase totals = $609.92       $665.9447   $665.94   $665.90   $666.00