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 $20.28   5.225%   $21.3396   $21.34   $21.35   $21.30  
  Purchase #2 $122.40   12.454%   $137.6437   $137.64   $137.65   $137.60  
  Purchase #3 $114.92   5.509%   $121.2509   $121.25   $121.25   $121.30  
  Purchase #4 $37.46   11.417%   $41.7368   $41.74   $41.75   $41.70  
  Purchase #5 $227.67   9.680%   $249.7085   $249.71   $249.70   $249.70  
  Purchase #6 $178.30   5.685%   $188.4364   $188.44   $188.45   $188.40  
  Purchase #7 $134.65   7.158%   $144.2882   $144.29   $144.30   $144.30  
  7 purchase totals = $835.68       $904.4041   $904.41   $904.45   $904.30