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 $23.80   4.347%   $24.8346   $24.83   $24.85   $24.80  
  Purchase #2 $9.60   4.106%   $9.9942   $9.99   $10.00   $10.00  
  Purchase #3 $72.42   8.629%   $78.6691   $78.67   $78.65   $78.70  
  Purchase #4 $129.60   8.179%   $140.2   $140.20   $140.20   $140.20  
  Purchase #5 $35.62   12.107%   $39.9325   $39.93   $39.95   $39.90  
  Purchase #6 $146.83   3.104%   $151.3876   $151.39   $151.40   $151.40  
  Purchase #7 $31.74   12.788%   $35.7989   $35.80   $35.80   $35.80  
  7 purchase totals = $449.61       $480.8169   $480.81   $480.85   $480.80