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 $249.43   11.755%   $278.7505   $278.75   $278.75   $278.80  
  Purchase #2 $104.81   10.385%   $115.6945   $115.69   $115.70   $115.70  
  Purchase #3 $227.82   10.104%   $250.8389   $250.84   $250.85   $250.80  
  Purchase #4 $177.39   12.663%   $199.8529   $199.85   $199.85   $199.90  
  Purchase #5 $64.83   3.671%   $67.2099   $67.21   $67.20   $67.20  
  Purchase #6 $9.25   12.456%   $10.4022   $10.40   $10.40   $10.40  
  Purchase #7 $68.96   6.266%   $73.281   $73.28   $73.30   $73.30  
  7 purchase totals = $902.49       $996.0299   $996.02   $996.05   $996.10