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 $52.51   9.510%   $57.5037   $57.50   $57.50   $57.50  
  Purchase #2 $230.79   7.719%   $248.6047   $248.60   $248.60   $248.60  
  Purchase #3 $208.32   7.390%   $223.7148   $223.71   $223.70   $223.70  
  Purchase #4 $95.69   12.806%   $107.9441   $107.94   $107.95   $107.90  
  Purchase #5 $133.90   5.118%   $140.753   $140.75   $140.75   $140.80  
  Purchase #6 $19.79   5.854%   $20.9485   $20.95   $20.95   $20.90  
  Purchase #7 $175.32   10.146%   $193.108   $193.11   $193.10   $193.10  
  7 purchase totals = $916.32       $992.5768   $992.56   $992.55   $992.50