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 $73.97   10.294%   $81.5845   $81.58   $81.60   $81.60  
  Purchase #2 $249.53   4.813%   $261.5399   $261.54   $261.55   $261.50  
  Purchase #3 $33.45   3.138%   $34.4997   $34.50   $34.50   $34.50  
  Purchase #4 $15.45   5.670%   $16.326   $16.33   $16.35   $16.30  
  Purchase #5 $73.58   9.318%   $80.4362   $80.44   $80.45   $80.40  
  Purchase #6 $184.96   6.446%   $196.8825   $196.88   $196.90   $196.90  
  Purchase #7 $93.76   13.694%   $106.5995   $106.60   $106.60   $106.60  
  7 purchase totals = $724.70       $777.8683   $777.87   $777.95   $777.80