I don’t think going into the tube flaccid makes any real difference, it’s just easier to get a good seal if you’re erect or semi erect and there’s less chance of getting loose skin sucked in.
You’ll still feel a difference though between getting a natural erection and adding a bit of pressure and using the negative pressure to get hard, a 100% real erection wins every time.
You want the feeling of the best 100% erection you’ve ever had rather than a 80% erection with the vacuum pulling on it if that makes sense.