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 $249.54   6.589%   $265.9822   $265.98   $266.00   $266.00  
  Purchase #2 $124.79   3.103%   $128.6622   $128.66   $128.65   $128.70  
  Purchase #3 $152.81   8.576%   $165.915   $165.92   $165.90   $165.90  
  Purchase #4 $120.45   4.627%   $126.0232   $126.02   $126.00   $126.00  
  Purchase #5 $62.57   12.725%   $70.532   $70.53   $70.55   $70.50  
  Purchase #6 $97.69   4.120%   $101.7148   $101.71   $101.70   $101.70  
  Purchase #7 $76.32   3.487%   $78.9813   $78.98   $79.00   $79.00  
  7 purchase totals = $884.17       $937.8107   $937.80   $937.80   $937.80