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 $19.75   8.735%   $21.4752   $21.48   $21.50   $21.50  
  Purchase #2 $121.34   6.694%   $129.4625   $129.46   $129.45   $129.50  
  Purchase #3 $142.42   11.656%   $159.0205   $159.02   $159.00   $159.00  
  Purchase #4 $67.77   12.210%   $76.0447   $76.04   $76.05   $76.00  
  Purchase #5 $120.62   6.334%   $128.2601   $128.26   $128.25   $128.30  
  Purchase #6 $191.88   5.562%   $202.5524   $202.55   $202.55   $202.60  
  Purchase #7 $108.54   5.622%   $114.6421   $114.64   $114.65   $114.60  
  7 purchase totals = $772.32       $831.4575   $831.45   $831.45   $831.50