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 $17.69   6.401%   $18.8223   $18.82   $18.80   $18.80  
  Purchase #2 $26.75   13.734%   $30.4238   $30.42   $30.40   $30.40  
  Purchase #3 $7.52   11.448%   $8.3809   $8.38   $8.40   $8.40  
  Purchase #4 $208.75   9.267%   $228.0949   $228.09   $228.10   $228.10  
  Purchase #5 $130.65   7.104%   $139.9314   $139.93   $139.95   $139.90  
  Purchase #6 $103.88   11.853%   $116.1929   $116.19   $116.20   $116.20  
  Purchase #7 $6.64   4.194%   $6.9185   $6.92   $6.90   $6.90  
  7 purchase totals = $501.88       $548.7647   $548.75   $548.75   $548.70