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 $78.29   3.825%   $81.2846   $81.28   $81.30   $81.30  
  Purchase #2 $143.34   13.281%   $162.377   $162.38   $162.40   $162.40  
  Purchase #3 $114.40   13.116%   $129.4047   $129.40   $129.40   $129.40  
  Purchase #4 $25.38   9.208%   $27.717   $27.72   $27.70   $27.70  
  Purchase #5 $44.85   10.528%   $49.5718   $49.57   $49.55   $49.60  
  Purchase #6 $89.64   8.663%   $97.4055   $97.41   $97.40   $97.40  
  Purchase #7 $78.55   3.397%   $81.2183   $81.22   $81.20   $81.20  
  7 purchase totals = $574.45       $628.9789   $628.98   $628.95   $629.00