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 $19.31   9.485%   $21.1416   $21.14   $21.15   $21.10  
  Purchase #2 $19.99   3.170%   $20.6237   $20.62   $20.60   $20.60  
  Purchase #3 $153.77   9.391%   $168.2105   $168.21   $168.20   $168.20  
  Purchase #4 $105.29   7.534%   $113.2225   $113.22   $113.20   $113.20  
  Purchase #5 $241.48   9.529%   $264.4906   $264.49   $264.50   $264.50  
  Purchase #6 $123.58   10.158%   $136.1333   $136.13   $136.15   $136.10  
  Purchase #7 $16.71   13.389%   $18.9473   $18.95   $18.95   $18.90  
  7 purchase totals = $680.13       $742.7695   $742.76   $742.75   $742.60