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 $16.99   9.698%   $18.6377   $18.64   $18.65   $18.60  
  Purchase #2 $155.46   6.768%   $165.9815   $165.98   $166.00   $166.00  
  Purchase #3 $181.76   10.210%   $200.3177   $200.32   $200.30   $200.30  
  Purchase #4 $176.70   12.316%   $198.4624   $198.46   $198.45   $198.50  
  Purchase #5 $101.98   12.534%   $114.7622   $114.76   $114.75   $114.80  
  Purchase #6 $6.82   4.440%   $7.1228   $7.12   $7.10   $7.10  
  Purchase #7 $69.58   9.525%   $76.2075   $76.21   $76.20   $76.20  
  7 purchase totals = $709.29       $781.4918   $781.49   $781.45   $781.50