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 $159.81   13.568%   $181.493   $181.49   $181.50   $181.50  
  Purchase #2 $35.45   3.775%   $36.7882   $36.79   $36.80   $36.80  
  Purchase #3 $46.52   12.163%   $52.1782   $52.18   $52.20   $52.20  
  Purchase #4 $234.59   6.259%   $249.273   $249.27   $249.25   $249.30  
  Purchase #5 $186.33   13.515%   $211.5125   $211.51   $211.50   $211.50  
  Purchase #6 $66.41   10.833%   $73.6042   $73.60   $73.60   $73.60  
  Purchase #7 $30.49   11.595%   $34.0253   $34.03   $34.05   $34.00  
  7 purchase totals = $759.60       $838.8744   $838.87   $838.90   $838.90