"Toons".

I don't think there's a "wrong" time to do it (except, probably, as soon as you have mana for the bill).

I personally don't bother with it for the most part. For the main story, I just used the first tiers. After I was done with it, I only reincarnated my main spellcasters (to get the higher tier spells) and thieves. Way later on, I created a humanoid and a monster generic just to unlock all tiers in all classes/species (used them + Zetta on CoO4), so I only had to upgrade the original generics once. The last time I reincarnated, after another and even longer stretch, was to farm stored levels, basically the last "important" thing I did in my file (by then I already had all achievements, but I still wanted to max out my characters).