Also because I feel more comfortable having icon slots free for future use than filling them all up right now and then having to swap out icons later. Yes, I know, some people would call me silly.

Not silly at all. That's exactly the reason why I haven't filled up all my icon slots yet. What if I discover a new fandom and have no slots free to upload new icons? I'd have to delete old ones (that I might still want to use!). So yeah, not filling up now.