I understand that it is not "necessary" to treat preventatively, however, If I do so, will this eliminate the possibility that she will have any symptoms? Will it do any good - or will it not make any difference?
Yes, I understand that there are side effects to every medicine. My daughter basically shared bed pillows with her infected cousin, and I am afraid the lice or nits have very likely been transferred.
If the medicine would work to kill lice/nits that were potentially transferred within one day, then it would be worth it to do it now and not waiting until she shows signs of infestation (and infects her brother, myself and others in the household). If you say that the medicine will not work until she has certain symptoms, that is a different story.