One of the reasons I always temped in the 2WW was because I rarely actually ovulated, so if FF gave me CH's, I couldn't always trust it. Sure enough, two or three times FF took away my CHs at around 9 or 10DPO because it became obvious that I did not in fact O. Of course I would have figured it out eventually when my period didn't show and I got BFN's, but it kept me more sane to temp in the 2WW.
Even for those that aren't as irregular as I was, sometimes FF will move your CHs. So if you don't temp and you think you are 10DPO when really you are only 7DPO because FF decided your O date was wrong, that could make for some really stressful days.