If you're picky about the time of day you take your test, then you should sign up early. The prime slots (late morning) fill up fast. On the other hand, if you're a morning person and want an 8 am test, you can usually sign up with a week's notice.

Also, if you need to take your test on a weekend (some test centers are open on Saturdays) you should sign up early since those fill up quickly, too.

From a prep standpoint, sometimes it's good to finally set the date so that you have a little pressure to kick your study into an even higher gear. If you've already been studying for six months, I think you might do well with a deadline.