I say it all the time to my ENTP. There doesn't have to be a big reason for me to say it, so I say it often. I am also very affectionate. I am a woman though, I don't know if there is a difference between male ISTJs and female ISTJs in terms of showing affection. Most of the (few) ISTJs in this forum are men and I think this sometimes does not give the whole picture.