I also agree that mineral oil does not evaporate at room temperature and at higher temperatures evaporation would be somewhat negligible if any. The boiling temperature for mineral oil is about 650 F and obviously is lower than its flash point. As you know, this is well above the operating temps of a pc and the oil will likely never reach that temperature when only used at this capacity.
However, mineral oil does have a relatively high absorption rate and I suspect that this may be why some people believe that it evaporates. This is actually one of the benefits of it. You can test it for yourself however and leave a container of it sitting and watch how quickly it evaporates. Typically the higher the temperature, the more the substance will evaporate.
In any case, if you did use mineral oil to cool your computer, evaporation would be noticeable once the oil level drops. You can tell how much the oil has evaporated by the amount left over time. Although there have been reports of oil loss by a wicking effect through cables immersed in the fluid. (Something to watch out for.)
As an example of how safe it is as compared to kerosene or some other petroleum distillate product...although I personally don't use it on my skin, mineral oil is basically baby oil with fragrance and other minor things added. It is considered safe for skin application. The simplest baby oil will be mineral oil and a fragrance of some sort. Certain brands may add other items just as any other product, but the main ingredient is mineral oil.