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 $104.50   12.162%   $117.2093   $117.21   $117.20   $117.20  
  Purchase #2 $85.69   12.435%   $96.3456   $96.35   $96.35   $96.30  
  Purchase #3 $93.57   11.585%   $104.4101   $104.41   $104.40   $104.40  
  Purchase #4 $223.65   11.574%   $249.5353   $249.54   $249.55   $249.50  
  Purchase #5 $35.85   9.350%   $39.202   $39.20   $39.20   $39.20  
  Purchase #6 $21.41   3.105%   $22.0748   $22.07   $22.05   $22.10  
  Purchase #7 $186.61   3.551%   $193.2365   $193.24   $193.25   $193.20  
  7 purchase totals = $751.28       $822.0136   $822.02   $822.00   $821.90