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 $40.97   10.838%   $45.4103   $45.41   $45.40   $45.40  
  Purchase #2 $19.81   5.802%   $20.9594   $20.96   $20.95   $21.00  
  Purchase #3 $54.57   6.175%   $57.9397   $57.94   $57.95   $57.90  
  Purchase #4 $25.45   11.272%   $28.3187   $28.32   $28.30   $28.30  
  Purchase #5 $101.98   12.398%   $114.6235   $114.62   $114.60   $114.60  
  Purchase #6 $129.50   6.265%   $137.6132   $137.61   $137.60   $137.60  
  Purchase #7 $238.34   7.762%   $256.84   $256.84   $256.85   $256.80  
  7 purchase totals = $610.62       $661.7048   $661.70   $661.65   $661.60