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 $244.95   10.764%   $271.3164   $271.32   $271.30   $271.30  
  Purchase #2 $227.69   3.143%   $234.8463   $234.85   $234.85   $234.80  
  Purchase #3 $47.67   5.338%   $50.2146   $50.21   $50.20   $50.20  
  Purchase #4 $53.46   6.323%   $56.8403   $56.84   $56.85   $56.80  
  Purchase #5 $169.80   12.568%   $191.1405   $191.14   $191.15   $191.10  
  Purchase #6 $25.53   12.328%   $28.6773   $28.68   $28.70   $28.70  
  Purchase #7 $47.25   7.638%   $50.859   $50.86   $50.85   $50.90  
  7 purchase totals = $816.35       $883.8944   $883.90   $883.90   $883.80