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 $240.54   10.614%   $266.0709   $266.07   $266.05   $266.10  
  Purchase #2 $189.26   5.132%   $198.9728   $198.97   $198.95   $199.00  
  Purchase #3 $196.59   4.252%   $204.949   $204.95   $204.95   $204.90  
  Purchase #4 $77.99   12.432%   $87.6857   $87.69   $87.70   $87.70  
  Purchase #5 $132.35   5.186%   $139.2137   $139.21   $139.20   $139.20  
  Purchase #6 $69.35   6.293%   $73.7142   $73.71   $73.70   $73.70  
  Purchase #7 $30.37   3.242%   $31.3546   $31.35   $31.35   $31.40  
  7 purchase totals = $936.45       $1,001.9609   $1,001.95   $1,001.90   $1,002.00