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 $222.74   7.576%   $239.6148   $239.61   $239.60   $239.60  
  Purchase #2 $51.92   12.577%   $58.45   $58.45   $58.45   $58.50  
  Purchase #3 $60.25   7.167%   $64.5681   $64.57   $64.55   $64.60  
  Purchase #4 $169.98   3.804%   $176.446   $176.45   $176.45   $176.40  
  Purchase #5 $128.86   5.835%   $136.379   $136.38   $136.40   $136.40  
  Purchase #6 $57.66   3.398%   $59.6193   $59.62   $59.60   $59.60  
  Purchase #7 $144.38   9.706%   $158.3935   $158.39   $158.40   $158.40  
  7 purchase totals = $835.79       $893.4707   $893.47   $893.45   $893.50