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 $18.30   13.496%   $20.7698   $20.77   $20.75   $20.80  
  Purchase #2 $12.33   12.692%   $13.8949   $13.89   $13.90   $13.90  
  Purchase #3 $115.61   5.343%   $121.787   $121.79   $121.80   $121.80  
  Purchase #4 $243.66   7.557%   $262.0734   $262.07   $262.05   $262.10  
  Purchase #5 $66.25   13.495%   $75.1904   $75.19   $75.20   $75.20  
  Purchase #6 $193.88   4.816%   $203.2173   $203.22   $203.20   $203.20  
  Purchase #7 $38.86   4.554%   $40.6297   $40.63   $40.65   $40.60  
  7 purchase totals = $688.89       $737.5625   $737.56   $737.55   $737.60