I think that thought Welcome bags are definitely optional, it is definitely at nice gesture for your guests. It is one of those things that you need to make room for in your budget, because we are asking a lot of our family and friends to be there! If you think that your guests wont use the items that you are purchasing for the bags, then don't put them in the bags! I am only choosing items that I know will be useful for my guests.
In terms of personalizing the bags, I won't be making them different from one another, but I will attach a separate thank you card addressed to each individual.
But in the end, it is up to you, and what you think works best for your family