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 $7.60   10.812%   $8.4217   $8.42   $8.40   $8.40  
  Purchase #2 $142.93   7.805%   $154.0857   $154.09   $154.10   $154.10  
  Purchase #3 $114.35   5.306%   $120.4174   $120.42   $120.40   $120.40  
  Purchase #4 $90.81   3.786%   $94.2481   $94.25   $94.25   $94.20  
  Purchase #5 $225.32   10.599%   $249.2017   $249.20   $249.20   $249.20  
  Purchase #6 $143.68   4.513%   $150.1643   $150.16   $150.15   $150.20  
  Purchase #7 $117.32   10.470%   $129.6034   $129.60   $129.60   $129.60  
  7 purchase totals = $842.01       $906.1423   $906.14   $906.10   $906.10