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 $17.72   13.356%   $20.0867   $20.09   $20.10   $20.10  
  Purchase #2 $151.25   5.774%   $159.9832   $159.98   $160.00   $160.00  
  Purchase #3 $51.69   11.356%   $57.5599   $57.56   $57.55   $57.60  
  Purchase #4 $129.27   13.217%   $146.3556   $146.36   $146.35   $146.40  
  Purchase #5 $127.67   10.785%   $141.4392   $141.44   $141.45   $141.40  
  Purchase #6 $6.39   8.582%   $6.9384   $6.94   $6.95   $6.90  
  Purchase #7 $21.33   9.705%   $23.4001   $23.40   $23.40   $23.40  
  7 purchase totals = $505.32       $555.7631   $555.77   $555.80   $555.80