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 $13.42   11.767%   $14.9991   $15.00   $15.00   $15.00  
  Purchase #2 $239.32   8.480%   $259.6143   $259.61   $259.60   $259.60  
  Purchase #3 $21.34   7.717%   $22.9868   $22.99   $23.00   $23.00  
  Purchase #4 $13.61   6.402%   $14.4813   $14.48   $14.50   $14.50  
  Purchase #5 $128.46   8.781%   $139.7401   $139.74   $139.75   $139.70  
  Purchase #6 $35.39   7.635%   $38.092   $38.09   $38.10   $38.10  
  Purchase #7 $65.77   3.764%   $68.2456   $68.25   $68.25   $68.20  
  7 purchase totals = $517.31       $558.1592   $558.16   $558.20   $558.10