If you're in a 10 team league, you're bound to get a top 10 Def, unless you have a lot of beginners wasting picks by choosing 2 Def's - if that happens ya may have to grab one a couple rounds earlier then the end. What I generally do is decide on my Def rankings and from that come up with a list of Def's that I consider acceptable to draft - if I have 10-12 on that list then I know I don't have to worry until the end - if I only had 8 on that list that I felt strongly about, I would have to keep an eye on that list in the latter rounds once it gets down to about 5 or 6 gone from that list to determine if I should grab my Def a little earlier than planned rather than miss out on one of those, taking into consideration my needs at other positions and what is left on the board at other positions.
Rather decide how high to rank various players at various positions in relation to each other, I simply rank all the QB's together, then the RB's, WR's, and so on - takes much less time to pre-rank (but only do this for a live draft, and make sure ya show up for it if ya do it this way, otherwise ya won't get great picks). When you draft you can just just switch back and forth between positions - to decide whether to draft a certain position, or take the best player available (just make sure when you enter the draft room you switch from Yahoo's rankings to your rankings - you'll see drop down box there to do that). And you are limited to how many players you can pre-rank on Yahoo - I believe it's 10 per roster spot, so if you have a 15 man roster (including bench spots), you can pre-rank 150 players only. Sometimes if my list is too long, I don't even pre-rank the kickers, I just keep a list of them pre-ranked on paper in front of me to use when the last round comes along.
Make sure ya read as much as ya can on this board before ya draft - all kinds of great info here. And don't be afraid to ask questions!
Fantasy Football: "Luck is where preparation meets opportunity"