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 $27.39   9.687%   $30.0433   $30.04   $30.05   $30.00  
  Purchase #2 $74.59   9.687%   $81.8155   $81.82   $81.80   $81.80  
  Purchase #3 $69.95   4.366%   $73.004   $73.00   $73.00   $73.00  
  Purchase #4 $113.58   6.687%   $121.1751   $121.18   $121.20   $121.20  
  Purchase #5 $125.40   13.288%   $142.0632   $142.06   $142.05   $142.10  
  Purchase #6 $90.61   7.204%   $97.1375   $97.14   $97.15   $97.10  
  Purchase #7 $28.55   13.162%   $32.3078   $32.31   $32.30   $32.30  
  7 purchase totals = $530.07       $577.5464   $577.55   $577.55   $577.50