Tim,
I have use a lot of jatoba, and have never found the need to use a grain filler. I do use oil finishes most of the time, but I don't really consider jatoba to have a very open grain. It may not be like maple or cherry, but on the other hand, it is not open like oak either. I think it really depends upon what you are looking for in the finished piece. If you want a high gloss, mirror finish, then a filler would certainly be worth consideration. If you want the other extreme of a more natural looking surface, then a filler would not be needed. I would suggest doing a couple of samples, one with a filler and one without, using the same finish schedule otherwise. Let us know what you decide upon, and post some pictures of the finished piece. :thumbs_up
Hope this helps.
Matt