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 $18.83   11.461%   $20.9881   $20.99   $21.00   $21.00  
  Purchase #2 $43.32   9.639%   $47.4956   $47.50   $47.50   $47.50  
  Purchase #3 $153.44   12.420%   $172.4972   $172.50   $172.50   $172.50  
  Purchase #4 $187.86   13.643%   $213.4897   $213.49   $213.50   $213.50  
  Purchase #5 $143.73   5.526%   $151.6725   $151.67   $151.65   $151.70  
  Purchase #6 $133.86   8.527%   $145.2742   $145.27   $145.25   $145.30  
  Purchase #7 $151.68   11.815%   $169.601   $169.60   $169.60   $169.60  
  7 purchase totals = $832.72       $921.0183   $921.02   $921.00   $921.10