My $$$ League uses 3 IDPs and I love it. We're allowed to use players from any defensive position so people usually take LBs...with the exception of the elite safeties like Ed Reed, Roy Williams, and Adrian Wilson. The best time to start drafting them is when the first one goes off the board, which is usually 4-5 rounds before the draft is over. There's practically no difference between the top 10 LBs point-wise, which is why you don't need to be the first to pick one. Just don't waste a pick on Ray Lewis...
I used to be in a league where we started 11 IDP's. Usually you just want players that get a bunch of tackles and there isn't much of a drop off between who you might think should be picked high and others. Don't worry about it until late in the draft.
There really isn't anywhre to mock draft IDPs. They're kinda like the underground hardcore fantasyfootball owner's favorite secret, which sucks because mainly great leagues get into them, but there isn't much support out there for them.
There are plenty of posts here on basic guidelines to follow, but alot of the info is dependent. For starters, unlike offense, there is no standard scoring scheme (like 25/4, 10/6) nor is there a standard lineup (QB, 2 RB, 3 WR, TE, K), so IDP rankings are very hard to come by. When to draft depends alot on how many guys you start in IDP. If its 1-3, wait awhile, if its like 6-7, mid-late rounds might be a good time.
For positions, generally:
LB > DB > DL
LB: Tackles are the key to most IDPs, higher tackle totals, usually mean higher scorers as all the other stats are too fluky. LBs tackle more than anyone, thus they are IDP's RBs. MLBs get the most tackles, so 4-3 MLBs are tops. Next come the 4-3 WLBs who the top ones can usually outproduce a few MLBs. Then you have your ILBs and then OLBs in 3-4. ILBs tend to make more tackles (Farrior, Bruschi, Edwards, etc). Last is your SLBs. These guys generally have to take on fullbacks and other lead blockers so the MLB can make tackles, hence don't translate into good IDPs.
DBs: If LBs are RBs, these guys are WRs. Safeties make more tackles usually, hence they have preference, and strong safeties are involved in stopping the run even more, thus SS is tops in this group. Then FS, as the really good ones tend to play FS like a SS (Roy Williams, Brian Dawkins, etc). Next are your rookie CBs or weak #2 CBs. You actually want players that aren't very good football players, b/c if they're not covering a guy well, they'll get thrown at often and end up with higher tackle totals, PDs, and INTs just out of opportunity. Lastly are the cover CBs, guys like Champ Bailey just tend to make QBs throw away from them, nullifying their IDP value. (Also look for the CBs with weak OLBs, as they will likely have to support the run more).
DLs: DLs are like Kickers sadly. They're inconsistent, and not half as important as good LBs or DBs. They make the least tackles, thus usually the worst scorers on your team. There are a few gems, but generally treat them like kickers, and avoid just about every DT. DEs have value because they sack people and make tackles on the outside. But DTs primarily stuff the middle by taking on blockers. That doesnt give them any fantasy points though.
Thanks alot you guys. This is my roster in this 20 round, 5 team, 17 starter (1 QB, 2 RBs, 3 WRs, 1 RB/WR/TE flex, 1 TE, 1 K, 2 DBs, 3 DLs, 3 DB/DL/LB flexes) league with 3 bench spots. How do you guys think I did? I had 4th pick.
QB - Tom Brady
RB - LaDainian Tomlinson, Rudi Johnson
RB/WR/TE Flex - Edgerrin James
WR - Torry Holt, Anquan Boldin, Chris Chambers
TE - Jeremy Shockey
K - Lawrence Tynes
DB - Adam Archuleta, Ronde Barber
DL - Adalius Thomas, Michael Strahan, Charles Grant
DB/DL/LB Flex - Brian Urlacher, Jeremiah Trotter, Andra Davis
BN - Drew Brees, Randy Moss, Brian Westbrook
This will probably be my only IDP league, I just wanted to know what it was like drafting in an IDP, and I don't like it too much.
Made some roster changes, the commish added more spots. Picked up Crumpler, Tedy Bruschi. Crumpler is also on my bench and Bruschi starts instead of Jeremiah (I think I have an unfair commish who took Jeremiah off my team), is that any better? And should I pick up Madieu for Ronde? And maybe Dan Morgan for Tedy?