These two positions are difficult to project because of how little separation there is between individual players. Kickers and defenses fluctuate slightly over the years, but as long as you aren't playing Scobee you should be fine. That is an interesting point, actually. Scobee is an incredible kicker, but the number of times that the Jags offense is going to move the ball into field goal range is minimal. Typically, you should draft kickers from teams with dynamic offenses. Ones that will have ample opportunities to kick field goals. The two exceptions to this rule last year were David Akers and Janikowski. Akers had an unbelievable season because the 49ers just never turned the ball over. Janikowski had a great season because he is an amazing kicker. You are taking a risk on consistency with players who aren't going to score points on extra points, but it can pay off. For the most part, my recommendation is to pick a kicker like Mason Crosby, of the Packers, or Gostkowski of the Patriots.
Defense is similarly difficult to predict. So much of defense rests on the ability to generate turnovers and limit offenses to abnormally low point totals that it is difficult to predict which teams will shine each year. We already know that the Packers will get a lot of turnovers, that's just what they do, and that the Ravens will limit teams' point totals, but the question is really about whether any team can do both. The Titans or Texans could have surprisingly good defensive showings this year thanks to their in-division games. Alternatively, the Vikings are destined to have a horrendous season playing in the NFC North. Make a gut decision on this one, and you can also pick up a better defense on FA or during bye weeks.
If you have an elite kicker or defense, do not drop them for their bye week. I went into bye weeks with mediocre kickers and defenses and came out with top 5 guys. It might not make a huge difference, but when I can score an extra 5 points per week with my kicker-defense combination, it really adds up. They are valuable enough to draft, they are valuable enough to hold during byes.