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 $28.93   13.404%   $32.8078   $32.81   $32.80   $32.80  
  Purchase #2 $186.95   3.597%   $193.6746   $193.67   $193.65   $193.70  
  Purchase #3 $124.63   4.312%   $130.004   $130.00   $130.00   $130.00  
  Purchase #4 $166.43   7.626%   $179.122   $179.12   $179.10   $179.10  
  Purchase #5 $5.69   5.280%   $5.9904   $5.99   $6.00   $6.00  
  Purchase #6 $23.48   6.108%   $24.9142   $24.91   $24.90   $24.90  
  Purchase #7 $182.31   13.445%   $206.8216   $206.82   $206.80   $206.80  
  7 purchase totals = $718.42       $773.3346   $773.32   $773.25   $773.30