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 $53.67   11.851%   $60.0304   $60.03   $60.05   $60.00  
  Purchase #2 $179.54   8.509%   $194.8171   $194.82   $194.80   $194.80  
  Purchase #3 $19.26   6.138%   $20.4422   $20.44   $20.45   $20.40  
  Purchase #4 $213.41   12.185%   $239.414   $239.41   $239.40   $239.40  
  Purchase #5 $107.80   13.367%   $122.2096   $122.21   $122.20   $122.20  
  Purchase #6 $6.39   5.406%   $6.7354   $6.74   $6.75   $6.70  
  Purchase #7 $129.86   3.765%   $134.7492   $134.75   $134.75   $134.70  
  7 purchase totals = $709.93       $778.3979   $778.40   $778.40   $778.20