@Mojo hi! That's a very good question. Well, it depends. Sometimes I do, sometimes I don't. Let me give you a few examples when I don't tag anyone, but still give credit.
On my personal accounts, I mostly share a lot of stuff related to the arts, but also quotes. What I usually do is giving credit either in the caption text (1st and 2nd images below), or "steal" an image which has its own credit (3rd and 4th images below).
For the first 2 images below, I only write "By NameArtist." And for the other 2 images, there's the "@account" of the original poster in the image.
Then, of course, if you want to give credit to make friends and/or attract the attention of the original poster, that's an entire different story.
Once again, it depends on the purpose of your act. For me, I just want to delight my followers, so I don't really care if the image owner knows about me posting his/her images. Nevertheless, I like giving credit to the image creator. But I still steal content ...
Hope that helps.
Ciao for now!