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 $28.91   5.627%   $30.5368   $30.54   $30.55   $30.50  
  Purchase #2 $114.78   9.179%   $125.3157   $125.32   $125.30   $125.30  
  Purchase #3 $19.72   9.840%   $21.6604   $21.66   $21.65   $21.70  
  Purchase #4 $51.55   13.488%   $58.5031   $58.50   $58.50   $58.50  
  Purchase #5 $7.53   3.600%   $7.8011   $7.80   $7.80   $7.80  
  Purchase #6 $146.69   13.325%   $166.2364   $166.24   $166.25   $166.20  
  Purchase #7 $181.34   13.342%   $205.5344   $205.53   $205.55   $205.50  
  7 purchase totals = $550.52       $615.5879   $615.59   $615.60   $615.50