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 $28.45   6.793%   $30.3826   $30.38   $30.40   $30.40  
  Purchase #2 $105.71   8.776%   $114.9871   $114.99   $115.00   $115.00  
  Purchase #3 $43.69   13.383%   $49.537   $49.54   $49.55   $49.50  
  Purchase #4 $52.33   6.774%   $55.8748   $55.87   $55.85   $55.90  
  Purchase #5 $89.85   4.499%   $93.8924   $93.89   $93.90   $93.90  
  Purchase #6 $62.99   13.312%   $71.3752   $71.38   $71.40   $71.40  
  Purchase #7 $230.56   7.752%   $248.433   $248.43   $248.45   $248.40  
  7 purchase totals = $613.58       $664.4821   $664.48   $664.55   $664.50