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 $18.90   8.263%   $20.4617   $20.46   $20.45   $20.50  
  Purchase #2 $140.99   5.360%   $148.5471   $148.55   $148.55   $148.50  
  Purchase #3 $122.96   9.678%   $134.8601   $134.86   $134.85   $134.90  
  Purchase #4 $56.95   12.776%   $64.2259   $64.23   $64.25   $64.20  
  Purchase #5 $100.86   13.748%   $114.7262   $114.73   $114.75   $114.70  
  Purchase #6 $83.62   12.592%   $94.1494   $94.15   $94.15   $94.10  
  Purchase #7 $121.71   11.260%   $135.4145   $135.41   $135.40   $135.40  
  7 purchase totals = $645.99       $712.3849   $712.39   $712.40   $712.30