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 $27.50   12.296%   $30.8814   $30.88   $30.90   $30.90  
  Purchase #2 $74.45   4.354%   $77.6916   $77.69   $77.70   $77.70  
  Purchase #3 $19.78   9.690%   $21.6967   $21.70   $21.70   $21.70  
  Purchase #4 $219.76   8.491%   $238.4198   $238.42   $238.40   $238.40  
  Purchase #5 $114.97   13.438%   $130.4197   $130.42   $130.40   $130.40  
  Purchase #6 $68.90   13.112%   $77.9342   $77.93   $77.95   $77.90  
  Purchase #7 $40.91   5.492%   $43.1568   $43.16   $43.15   $43.20  
  7 purchase totals = $566.27       $620.2002   $620.20   $620.20   $620.20