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 $28.90   4.150%   $30.0994   $30.10   $30.10   $30.10  
  Purchase #2 $25.44   13.791%   $28.9484   $28.95   $28.95   $28.90  
  Purchase #3 $131.53   10.423%   $145.2394   $145.24   $145.25   $145.20  
  Purchase #4 $46.39   13.659%   $52.7264   $52.73   $52.75   $52.70  
  Purchase #5 $64.89   3.198%   $66.9652   $66.97   $66.95   $67.00  
  Purchase #6 $177.60   6.293%   $188.7764   $188.78   $188.80   $188.80  
  Purchase #7 $40.45   3.419%   $41.833   $41.83   $41.85   $41.80  
  7 purchase totals = $515.20       $554.5882   $554.60   $554.65   $554.50