For me, the deciding factor on when to dual is how many levels of druid do I want her to have. I don't want to use up too much xp on the secondary class. Besides, while the fighter hitpoints are nice, if you're not planning on keeping her a fighter long enough to get extra attacks, they don't matter that much (imho). And if you do keep her a fighter that long, she's not going to be a very effective druid. Personally, I'm planning on switching her over once the group reaches Kuldahar since it seems to make sense story-wise and I won't need additional cleric magic before then.
On second thought - I checked an old IWD game and saw that my 6-person party had over 2 million experience points near the end of the game, so you can pretty much dual anytime south of 1 million xp and you should be okay.