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 $214.42   11.708%   $239.5243   $239.52   $239.50   $239.50  
  Purchase #2 $157.46   8.361%   $170.6252   $170.63   $170.65   $170.60  
  Purchase #3 $44.55   5.773%   $47.1219   $47.12   $47.10   $47.10  
  Purchase #4 $232.87   7.302%   $249.8742   $249.87   $249.85   $249.90  
  Purchase #5 $15.29   11.437%   $17.0387   $17.04   $17.05   $17.00  
  Purchase #6 $36.51   4.670%   $38.215   $38.22   $38.20   $38.20  
  Purchase #7 $67.71   4.712%   $70.9005   $70.90   $70.90   $70.90  
  7 purchase totals = $768.81       $833.2998   $833.30   $833.25   $833.20