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 $19.88   3.300%   $20.536   $20.54   $20.55   $20.50  
  Purchase #2 $155.93   12.490%   $175.4057   $175.41   $175.40   $175.40  
  Purchase #3 $25.72   5.395%   $27.1076   $27.11   $27.10   $27.10  
  Purchase #4 $13.27   12.158%   $14.8834   $14.88   $14.90   $14.90  
  Purchase #5 $120.73   3.472%   $124.9217   $124.92   $124.90   $124.90  
  Purchase #6 $105.80   9.837%   $116.2075   $116.21   $116.20   $116.20  
  Purchase #7 $94.99   4.860%   $99.6065   $99.61   $99.60   $99.60  
  7 purchase totals = $536.32       $578.6684   $578.68   $578.65   $578.60