Ordinary light pencil on the back of a photo printed on paper should not bleed through. Too much writing pressure will damage the paper and surface of the photo.
I wouldn't use ink. Have seen it bleed onto other photos.
There is a need to make things simple for later generations. If they can see who the persons are from the back of the photo without having to look other information that might get separated from the photos over time, that's very helpful.
There really isn't much room on the back of most photos to tell a story. Often, there are a series of photos over time that capture the most important parts of family history.
Most photo book printers of whom I am aware seem focused on photo printing and force you into their printing templates. I just wanted a simple way of tying our photos together with what I knew about our family history. So that's why I used MS Word along with a laser printer at home.
One method of dealing with information management pertaining to a very large number of photographs that have been scanned is to use a program like Photo Mechanic Plus. It is specifically made for captioning photos with IPTC data. It is typically used by sports photographers sending their photos out for news media publication. The news media can look at the embedded IPTC data and immediately know who's in the photo, what they were doing, when it was taken, who made the photo, etc.
Once names, dates, places are entered into the IPTC data for each photo, the program is very quick at sorting them. When you are trying to figure out who people are in photos, it's helpful to be able to compare photos over time to try to figure out the missing names. This kind of photo sorting is much faster than trying to sort through many, many paper photos. And it avoids potential damage by repeated handling of old photos.
I'm not advocating using digital media for long term storage of family photos, but I will say this program is excellent as a photo management tool.
New: Photo Mechanic Plus - Camera Bits, Inc..