While the top candidates are still involved with their final games of the season, a new appointment can only undermine their position and create a feeling of unrest at the chosen club.
Favourite Steve McClaren is focused on Boro's last two league games and the UEFA Cup Final, but you're not telling me that if he gets the job it won't affect him in any way.
Needless to say, the appointment could also undermine England's chances in the World Cup. In my opinion the FA should not be announcing anything until at least the end of the domestic season, or even after the World Cup.