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 $96.78   12.770%   $109.1388   $109.14   $109.15   $109.10  
  Purchase #2 $170.29   9.721%   $186.8439   $186.84   $186.85   $186.80  
  Purchase #3 $30.77   3.700%   $31.9085   $31.91   $31.90   $31.90  
  Purchase #4 $113.53   13.820%   $129.2198   $129.22   $129.20   $129.20  
  Purchase #5 $18.36   4.392%   $19.1664   $19.17   $19.15   $19.20  
  Purchase #6 $37.89   6.662%   $40.4142   $40.41   $40.40   $40.40  
  Purchase #7 $21.83   11.671%   $24.3778   $24.38   $24.40   $24.40  
  7 purchase totals = $489.45       $541.0694   $541.07   $541.05   $541.00