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 $56.25   6.655%   $59.9934   $59.99   $60.00   $60.00  
  Purchase #2 $243.55   4.111%   $253.5623   $253.56   $253.55   $253.60  
  Purchase #3 $92.89   8.628%   $100.9045   $100.90   $100.90   $100.90  
  Purchase #4 $122.97   13.269%   $139.2869   $139.29   $139.30   $139.30  
  Purchase #5 $240.50   8.219%   $260.2667   $260.27   $260.25   $260.30  
  Purchase #6 $241.93   11.654%   $270.1245   $270.12   $270.10   $270.10  
  Purchase #7 $146.83   13.698%   $166.9428   $166.94   $166.95   $166.90  
  7 purchase totals = $1,144.92       $1,251.0811   $1,251.07   $1,251.05   $1,251.10