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 $179.57   10.116%   $197.7353   $197.74   $197.75   $197.70  
  Purchase #2 $26.81   10.631%   $29.6602   $29.66   $29.65   $29.70  
  Purchase #3 $124.70   9.256%   $136.2422   $136.24   $136.25   $136.20  
  Purchase #4 $184.64   6.482%   $196.6084   $196.61   $196.60   $196.60  
  Purchase #5 $161.89   13.718%   $184.0981   $184.10   $184.10   $184.10  
  Purchase #6 $78.64   13.387%   $89.1675   $89.17   $89.15   $89.20  
  Purchase #7 $237.91   6.640%   $253.7072   $253.71   $253.70   $253.70  
  7 purchase totals = $994.16       $1,087.2189   $1,087.23   $1,087.20   $1,087.20