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 $28.99   4.309%   $30.2392   $30.24   $30.25   $30.20  
  Purchase #2 $81.44   10.846%   $90.273   $90.27   $90.25   $90.30  
  Purchase #3 $76.36   6.670%   $81.4532   $81.45   $81.45   $81.50  
  Purchase #4 $155.59   13.407%   $176.45   $176.45   $176.45   $176.50  
  Purchase #5 $240.95   8.319%   $260.9946   $260.99   $261.00   $261.00  
  Purchase #6 $178.71   12.363%   $200.8039   $200.80   $200.80   $200.80  
  Purchase #7 $39.51   5.103%   $41.5262   $41.53   $41.55   $41.50  
  7 purchase totals = $801.55       $881.7401   $881.73   $881.75   $881.80