I DEFINITLEY write in mine. When I hear or read something that to speaks to me about a Bible verse I definitely underline the verse(s) and make little notes to refresh my memory later on. I have to squeeze my notes in the margins and anywhere I can.
One of my nieces says that when I die she wants my Bible because I have so much stuff written in there.
Now my SIL will NOT write in her Bible even when we are at Bible Study and looking up verses and learning new things. I have to write notes if it touches me and seems really important.