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 $20.27   3.749%   $21.0299   $21.03   $21.05   $21.00  
  Purchase #2 $142.99   11.605%   $159.584   $159.58   $159.60   $159.60  
  Purchase #3 $182.61   12.361%   $205.1824   $205.18   $205.20   $205.20  
  Purchase #4 $92.86   13.835%   $105.7072   $105.71   $105.70   $105.70  
  Purchase #5 $30.68   8.857%   $33.3973   $33.40   $33.40   $33.40  
  Purchase #6 $61.84   3.617%   $64.0768   $64.08   $64.10   $64.10  
  Purchase #7 $109.59   4.739%   $114.7835   $114.78   $114.80   $114.80  
  7 purchase totals = $640.84       $703.7611   $703.76   $703.85   $703.80