As far as I know, you can accept a position up until the day before the competition starts. Usually offers only last 3 months (if i remember correctly). After that, they expire and you can't accept them. So if you don't accept any of the offers you have right now, by the time March rolls around, you won't have any from those teams.
Also, where you accept depends on how you like to play. You could accept the job to coach the country of your heritage/nationality, if they are in the game. If you want a challenge, accept a team that is weaker, and try and go far with them.
The benefit to selecting a team early gives you the opportunity to play around with your squad. See who you like/don't like playing. Also, you may have to still qualify for the next major tournament your team competes in. So you will have a factor in whether or not they qualify.
I usually like to acquire a few of the stars on my national team for my club team, just so I can keep tabs on their growth and playing time more closely (especially if they are a potential superstar).