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 $17.99   9.712%   $19.7372   $19.74   $19.75   $19.70  
  Purchase #2 $228.43   12.804%   $257.6782   $257.68   $257.70   $257.70  
  Purchase #3 $101.76   4.433%   $106.271   $106.27   $106.25   $106.30  
  Purchase #4 $16.43   10.400%   $18.1387   $18.14   $18.15   $18.10  
  Purchase #5 $123.51   6.426%   $131.4468   $131.45   $131.45   $131.40  
  Purchase #6 $230.81   12.164%   $258.8857   $258.89   $258.90   $258.90  
  Purchase #7 $101.62   4.837%   $106.5354   $106.54   $106.55   $106.50  
  7 purchase totals = $820.55       $898.6930   $898.71   $898.75   $898.60