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 $6.40   9.266%   $6.993   $6.99   $7.00   $7.00  
  Purchase #2 $30.25   13.191%   $34.2403   $34.24   $34.25   $34.20  
  Purchase #3 $223.38   12.468%   $251.231   $251.23   $251.25   $251.20  
  Purchase #4 $225.54   7.389%   $242.2052   $242.21   $242.20   $242.20  
  Purchase #5 $46.58   8.687%   $50.6264   $50.63   $50.65   $50.60  
  Purchase #6 $43.90   7.638%   $47.2531   $47.25   $47.25   $47.30  
  Purchase #7 $44.98   7.205%   $48.2208   $48.22   $48.20   $48.20  
  7 purchase totals = $621.03       $680.7698   $680.77   $680.80   $680.70