I never worry about a backup kicker. I sometimes just pick up whatever kicker I happen to like that week. I went through five or six last year. When the bye rolls around, pick up whoever looks most promising. Just like in the NFL...your kicker goes down, just pick up a new one. No stress on that one.
Defense is a little different. I'd say pretty much the same thing, don't worry about a backup D if you're happy with your 1st D. If you're not happy with your first D and you have the bench spot, I can see making a case for picking two sleeper D's and watching to see which one develops into a better unit. Really though, I'd rather have a RB or QB that had a shot at contributing when he's facing the right matchup than another D. But if you've got the room, why not? Right now, I need to pick up a free agent WR, but I'm going to wait a week or two so I actually have something to judge them on. In the meantime, I actually am carrying two D's, because they're both sleepers and I don't know which will actually be my starter. I'll wait a week or two, and dump one for whichever receiver has showed the most promise in the early weeks. However, if I saw somebody I wanted on the waiver's, I'd drop my second D in a heartbeat.
For the D and kicker, if you're just worried about the bye, stop worrying. You'll be able to pick up something on the waiver wire that can fill in for the bye week. Having two kickers is a total waste of a spot, so forget that. If you're happy with your first D, then don't worry about another D either. If you're not, you can play the field a bit until you find a D that you think can start for you. Or even just go week to week, picking whatever D has the best matchup. Like, whoever happens to be playing St. Louis or Detroit or something.