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 $59.25   12.136%   $66.4406   $66.44   $66.45   $66.40  
  Purchase #2 $210.37   8.271%   $227.7697   $227.77   $227.75   $227.80  
  Purchase #3 $125.57   3.846%   $130.3994   $130.40   $130.40   $130.40  
  Purchase #4 $37.84   11.782%   $42.2983   $42.30   $42.30   $42.30  
  Purchase #5 $44.36   13.503%   $50.3499   $50.35   $50.35   $50.30  
  Purchase #6 $235.98   5.296%   $248.4775   $248.48   $248.50   $248.50  
  Purchase #7 $46.57   6.830%   $49.7507   $49.75   $49.75   $49.80  
  7 purchase totals = $759.94       $815.4861   $815.49   $815.50   $815.50