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 $227.92   12.331%   $256.0248   $256.02   $256.00   $256.00  
  Purchase #2 $57.54   7.262%   $61.7186   $61.72   $61.70   $61.70  
  Purchase #3 $8.57   3.161%   $8.8409   $8.84   $8.85   $8.80  
  Purchase #4 $54.97   7.239%   $58.9493   $58.95   $58.95   $58.90  
  Purchase #5 $149.91   11.560%   $167.2396   $167.24   $167.25   $167.20  
  Purchase #6 $65.63   13.563%   $74.5314   $74.53   $74.55   $74.50  
  Purchase #7 $237.45   10.737%   $262.945   $262.95   $262.95   $262.90  
  7 purchase totals = $801.99       $890.2496   $890.25   $890.25   $890.00