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 $249.87   6.665%   $266.5238   $266.52   $266.50   $266.50  
  Purchase #2 $80.61   3.594%   $83.5071   $83.51   $83.50   $83.50  
  Purchase #3 $144.91   10.274%   $159.7981   $159.80   $159.80   $159.80  
  Purchase #4 $14.25   9.288%   $15.5735   $15.57   $15.55   $15.60  
  Purchase #5 $114.49   6.750%   $122.2181   $122.22   $122.20   $122.20  
  Purchase #6 $58.76   11.792%   $65.689   $65.69   $65.70   $65.70  
  Purchase #7 $48.91   7.872%   $52.7602   $52.76   $52.75   $52.80  
  7 purchase totals = $711.80       $766.0698   $766.07   $766.00   $766.10