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 $238.55   11.488%   $265.9546   $265.95   $265.95   $266.00  
  Purchase #2 $119.37   4.479%   $124.7166   $124.72   $124.70   $124.70  
  Purchase #3 $239.86   4.275%   $250.114   $250.11   $250.10   $250.10  
  Purchase #4 $114.73   6.766%   $122.4926   $122.49   $122.50   $122.50  
  Purchase #5 $8.71   12.533%   $9.8016   $9.80   $9.80   $9.80  
  Purchase #6 $48.32   13.262%   $54.7282   $54.73   $54.75   $54.70  
  Purchase #7 $91.96   12.450%   $103.409   $103.41   $103.40   $103.40  
  7 purchase totals = $861.50       $931.2166   $931.21   $931.20   $931.20