First ask yourself why you're taking engagement portraits. Are you planning on sending an engagement announcement to your local paper? In which case you need to schedule a shoot sooner rather than later. Are the shots going on save-the-date cards? Then you have a few months because you don’t have to send out STD cards until six to nine months before your wedding. Or are you just taking them because you want to have some nice photos to display in your home. In this case you can take them whenever you want -- and with the winter weather blowing in you may want to wait until spring or even summer arrives.
If you’re not in a rush then I would wait until you’ve booked a photographer for your wedding. Using your wedding photographer will give you a chance to get to know each other so on your wedding day you’re both a little more relaxed in front of his camera. You may also be able to get a deal on an engagement shoot since you’ve booked him for the wedding.
When you meet with prospective photographers ask them about including an engagement portrait session in your contract -- how much it will cost, how many prints you get and what they will cost and when he or she recommends doing them. You also want to see some of the engagement portrait sessions they’ve shot.