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 $242.68   9.852%   $266.5888   $266.59   $266.60   $266.60  
  Purchase #2 $160.31   11.447%   $178.6607   $178.66   $178.65   $178.70  
  Purchase #3 $223.63   7.390%   $240.1563   $240.16   $240.15   $240.20  
  Purchase #4 $193.89   3.143%   $199.984   $199.98   $200.00   $200.00  
  Purchase #5 $235.67   3.601%   $244.1565   $244.16   $244.15   $244.20  
  Purchase #6 $227.39   9.477%   $248.9398   $248.94   $248.95   $248.90  
  Purchase #7 $203.57   11.521%   $227.0233   $227.02   $227.00   $227.00  
  7 purchase totals = $1,487.14       $1,605.5094   $1,605.51   $1,605.50   $1,605.60