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 $37.85   5.309%   $39.8595   $39.86   $39.85   $39.90  
  Purchase #2 $15.43   3.364%   $15.9491   $15.95   $15.95   $15.90  
  Purchase #3 $109.42   7.688%   $117.8322   $117.83   $117.85   $117.80  
  Purchase #4 $203.26   9.379%   $222.3238   $222.32   $222.30   $222.30  
  Purchase #5 $84.60   10.399%   $93.3976   $93.40   $93.40   $93.40  
  Purchase #6 $35.66   10.397%   $39.3676   $39.37   $39.35   $39.40  
  Purchase #7 $9.73   4.625%   $10.18   $10.18   $10.20   $10.20  
  7 purchase totals = $495.95       $538.9098   $538.91   $538.90   $538.90