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 $21.57   10.275%   $23.7863   $23.79   $23.80   $23.80  
  Purchase #2 $28.77   4.223%   $29.985   $29.99   $30.00   $30.00  
  Purchase #3 $118.62   3.491%   $122.761   $122.76   $122.75   $122.80  
  Purchase #4 $225.85   4.369%   $235.7174   $235.72   $235.70   $235.70  
  Purchase #5 $191.29   4.748%   $200.3724   $200.37   $200.35   $200.40  
  Purchase #6 $20.39   5.795%   $21.5716   $21.57   $21.55   $21.60  
  Purchase #7 $72.63   9.410%   $79.4645   $79.46   $79.45   $79.50  
  7 purchase totals = $679.12       $713.6582   $713.66   $713.60   $713.80