Quick rundown:
With modern games, always just run at your native res to start. So 1440p. With a single GTX970, that might already not be enough for 60fps. See what your performance is like. Bumping to 4k is a very big jump in pixels needing to be rendered, and is extremely taxing on performance. You can only do this if you've got a *lot* of performance overhead, which you probably wont have for the vast majority of modern games. If you're tolerant of 30fps, then it might open up more options for you, but I'd trade 60fps over the IQ benefit any day. Even then, many games probably wont even run at a consistent 30fps at 4k with a single 970.
Stick with 1440p for the most part. DSR is only there to provide you options to play with, it is not meant to be something where you suddenly play every game at some higher resolution. If you've got a lot of performance overhead(you're running well over 60fps) in a specific game, then try bumping up the resolution. Doesn't need to be 4k, either. There are steps in between that might hit the right IQ/performance balance better. For instance, I run Dark Souls 2 at 1800p or something like that. 2160p(4k) is just a bit too much and I start to get more frequent framedrops.
Another thing to consider is that not every game scales the UI well with increased resolutions. Some games become downright unplayable at resolutions like 4k because the HUD/UI becomes way too small. So you obviously wouldn't want to use DSR in these cases.
Basically, what resolution you use should be chosen on an individual basis. Maybe some games you want to sacrifice some performance for extra IQ. Maybe a different game you don't. Maybe one game just doesn't play well with 4k. Etc etc
