When you get a girl's insta do you put it in yours and wait for them to follow back, or do you put yours in theirs so that you know they'll follow you?

My philosophy so far has been just to get theirs and if they follow you back then they actually like you, but I feel like for girls with a lot of followers they might forget and/or not care enough to follow back - feel like I might be losing some people potentially because of this.

Any thoughts?