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 $9.33   11.335%   $10.3876   $10.39   $10.40   $10.40  
  Purchase #2 $115.59   5.319%   $121.7382   $121.74   $121.75   $121.70  
  Purchase #3 $161.31   11.343%   $179.6074   $179.61   $179.60   $179.60  
  Purchase #4 $125.99   8.700%   $136.9511   $136.95   $136.95   $137.00  
  Purchase #5 $44.42   6.266%   $47.2034   $47.20   $47.20   $47.20  
  Purchase #6 $88.79   7.427%   $95.3844   $95.38   $95.40   $95.40  
  Purchase #7 $49.39   10.417%   $54.535   $54.54   $54.55   $54.50  
  7 purchase totals = $594.82       $645.8071   $645.81   $645.85   $645.80