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 $237.76   12.233%   $266.8452   $266.85   $266.85   $266.80  
  Purchase #2 $116.63   8.351%   $126.3698   $126.37   $126.35   $126.40  
  Purchase #3 $123.91   12.583%   $139.5016   $139.50   $139.50   $139.50  
  Purchase #4 $239.68   9.118%   $261.534   $261.53   $261.55   $261.50  
  Purchase #5 $109.90   9.821%   $120.6933   $120.69   $120.70   $120.70  
  Purchase #6 $11.38   8.128%   $12.305   $12.31   $12.30   $12.30  
  Purchase #7 $211.63   10.826%   $234.5411   $234.54   $234.55   $234.50  
  7 purchase totals = $1,050.89       $1,161.7900   $1,161.79   $1,161.80   $1,161.70