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 $128.79   7.535%   $138.4943   $138.49   $138.50   $138.50  
  Purchase #2 $222.37   6.700%   $237.2688   $237.27   $237.25   $237.30  
  Purchase #3 $73.87   6.758%   $78.8621   $78.86   $78.85   $78.90  
  Purchase #4 $20.89   11.382%   $23.2677   $23.27   $23.25   $23.30  
  Purchase #5 $54.58   6.553%   $58.1566   $58.16   $58.15   $58.20  
  Purchase #6 $154.38   3.296%   $159.4684   $159.47   $159.45   $159.50  
  Purchase #7 $129.99   10.584%   $143.7481   $143.75   $143.75   $143.70  
  7 purchase totals = $784.87       $839.2660   $839.27   $839.20   $839.40