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 $247.61   3.378%   $255.9743   $255.97   $255.95   $256.00  
  Purchase #2 $31.78   10.133%   $35.0003   $35.00   $35.00   $35.00  
  Purchase #3 $214.27   6.361%   $227.8997   $227.90   $227.90   $227.90  
  Purchase #4 $227.94   8.367%   $247.0117   $247.01   $247.00   $247.00  
  Purchase #5 $185.82   11.563%   $207.3064   $207.31   $207.30   $207.30  
  Purchase #6 $141.86   4.359%   $148.0437   $148.04   $148.05   $148.00  
  Purchase #7 $115.26   5.767%   $121.907   $121.91   $121.90   $121.90  
  7 purchase totals = $1,164.54       $1,243.1431   $1,243.14   $1,243.10   $1,243.10