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 $228.25   5.104%   $239.8999   $239.90   $239.90   $239.90  
  Purchase #2 $219.55   10.775%   $243.2065   $243.21   $243.20   $243.20  
  Purchase #3 $61.55   7.743%   $66.3158   $66.32   $66.30   $66.30  
  Purchase #4 $118.94   11.804%   $132.9797   $132.98   $133.00   $133.00  
  Purchase #5 $204.80   12.859%   $231.1352   $231.14   $231.15   $231.10  
  Purchase #6 $80.66   7.541%   $86.7426   $86.74   $86.75   $86.70  
  Purchase #7 $21.66   8.769%   $23.5594   $23.56   $23.55   $23.60  
  7 purchase totals = $935.41       $1,023.8391   $1,023.85   $1,023.85   $1,023.80