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 $186.36   12.252%   $209.1928   $209.19   $209.20   $209.20  
  Purchase #2 $26.42   12.171%   $29.6356   $29.64   $29.65   $29.60  
  Purchase #3 $151.90   7.247%   $162.9082   $162.91   $162.90   $162.90  
  Purchase #4 $221.93   7.107%   $237.7026   $237.70   $237.70   $237.70  
  Purchase #5 $130.37   9.387%   $142.6078   $142.61   $142.60   $142.60  
  Purchase #6 $100.85   12.474%   $113.43   $113.43   $113.45   $113.40  
  Purchase #7 $61.38   5.666%   $64.8578   $64.86   $64.85   $64.90  
  7 purchase totals = $879.21       $960.3348   $960.34   $960.35   $960.30