That's a good one. Say "I love you" to them because it makes them happy, and because you love them. You're not doing it because you want to, but you're doing it because you love to see them happy. Still, I'd still do spontaneous stuff. If I really admire a woman I'm with, I won't hold back from constantly reminding her how much I adore her.
To me, displays of affection just have to be perfect and done the right way. For me, it's about avoiding insincerity, carelessness, and thoughtlessness.
Where are these other ISTJs finding SO's without picking up a random person they barely care about? I must be missing something. ><
Even when I was in a relationship with an ESFJ, she said I Love You about 50 times more than me. When she said it, I just acted like it didn't happen. I guess I'm the same as other ISTJs. If I love you, saying I love you is kind of redundant. However, I have no problem if I'm doing something with my SO that just surprises me about her or our feelings are at an all time high, then I'd say it, because I'd really mean it.