The only time an fo/eo can affect the outcome of the soap is:
1. When you use too much it can be a very big irritant to the skin
2. If the fo has vanillin in it, the soap can turn brown
3. The fo or eo has different color constituents in it, can cause your soap to turn a different color (yellow, blue, brown, orange, green, etc)
If you're looking for info on how different fo's do in soaps, lotions, etc, I highly recommend The Soap Scent Review Board
http://soapscentreview.obisoap.ca/ (you need a paid email addy to sign up though & you have to email the admin of the board to sign up)
If you're looking to learn about the safety of EO's, then there are a lot of books that can help with that. (I haven't had my morning coffee yet, so I suddenly can't think of the titles LOL. I'll find them & post again in a bit)