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 $242.44   7.649%   $260.9842   $260.98   $261.00   $261.00  
  Purchase #2 $113.97   11.835%   $127.4583   $127.46   $127.45   $127.50  
  Purchase #3 $164.74   13.457%   $186.9091   $186.91   $186.90   $186.90  
  Purchase #4 $37.76   4.625%   $39.5064   $39.51   $39.50   $39.50  
  Purchase #5 $202.92   9.630%   $222.4612   $222.46   $222.45   $222.50  
  Purchase #6 $121.55   8.755%   $132.1917   $132.19   $132.20   $132.20  
  Purchase #7 $61.57   7.151%   $65.9729   $65.97   $65.95   $66.00  
  7 purchase totals = $944.95       $1,035.4838   $1,035.48   $1,035.45   $1,035.60