How should i use shen's ultimate?

I usually play Shen toplane and I'm wondering how to use his ultimate. It is on a pretty long cooldown and I'm wondering how and when I should use it. I like to use it whenever someone bot lane get's ganked, but people sometimes get mad at me when I do, saying I'm stealing their xp. Often, as soon as the gank is over, I recall then run back to top lane. But I'm wondering if there's a more efficient way to use it. I am mainly looking for uses during the laning phase but information on how to use it in teamfight phase would also be nice