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 $18.58   3.719%   $19.271   $19.27   $19.25   $19.30  
  Purchase #2 $24.47   8.767%   $26.6153   $26.62   $26.60   $26.60  
  Purchase #3 $196.43   7.783%   $211.7181   $211.72   $211.70   $211.70  
  Purchase #4 $240.58   8.874%   $261.9291   $261.93   $261.95   $261.90  
  Purchase #5 $97.43   4.132%   $101.4558   $101.46   $101.45   $101.50  
  Purchase #6 $43.86   9.726%   $48.1258   $48.13   $48.15   $48.10  
  Purchase #7 $245.89   13.420%   $278.8884   $278.89   $278.90   $278.90  
  7 purchase totals = $867.24       $948.0035   $948.02   $948.00   $948.00