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 $222.49   12.348%   $249.9631   $249.96   $249.95   $250.00  
  Purchase #2 $213.25   11.521%   $237.8185   $237.82   $237.80   $237.80  
  Purchase #3 $134.95   8.166%   $145.97   $145.97   $145.95   $146.00  
  Purchase #4 $226.51   8.627%   $246.051   $246.05   $246.05   $246.10  
  Purchase #5 $206.76   9.604%   $226.6172   $226.62   $226.60   $226.60  
  Purchase #6 $144.77   3.769%   $150.2264   $150.23   $150.25   $150.20  
  Purchase #7 $119.55   11.245%   $132.9934   $132.99   $133.00   $133.00  
  7 purchase totals = $1,268.28       $1,389.6396   $1,389.64   $1,389.60   $1,389.70