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 $49.25   13.681%   $55.9879   $55.99   $56.00   $56.00  
  Purchase #2 $232.84   5.350%   $245.2969   $245.30   $245.30   $245.30  
  Purchase #3 $178.85   9.465%   $195.7782   $195.78   $195.80   $195.80  
  Purchase #4 $47.79   3.454%   $49.4407   $49.44   $49.45   $49.40  
  Purchase #5 $34.48   3.422%   $35.6599   $35.66   $35.65   $35.70  
  Purchase #6 $248.34   5.678%   $262.4407   $262.44   $262.45   $262.40  
  Purchase #7 $145.69   6.177%   $154.6893   $154.69   $154.70   $154.70  
  7 purchase totals = $937.24       $999.2936   $999.30   $999.35   $999.30