I now have a 27" monitor with 5K resolution (219 PPI), and it really helps my images to look good, and many of the older images won't come close to filling the screen at even 100%, actually I could get close to showing two of my 300D images side by side at 100%. So now I will actually check output destined for the web, such as 1280px images for POTN at 200%, since that better represents the actual magnification that the image will be viewed at on screen on most "normal" resolution monitors. One nice thing about the high resolution monitor is that I am actually able to view many images in their entirety, without the need for interpolation.
What I would really like is an 8K monitor at about a 32" size, since that will actually be running at just about 300 PPI, which would mean that finally screen resolution was catching up with camera resolution. By my reckoning you should be able to display a 24 MP 3:2 ratio image at 100% on an 8K 16:9 screen without losing any pixels. My 5K display is only 288 pixels short of fitting my 15 MP images from my 50D vertically, although if you rearranged the total number of pixels available to a 3:2 ratio the 50D images still wouldn't quite fit.
When it comes to working in Ps though I will often work on the image at 800%, usually when painting in layer masks to ensure that they are pixel perfect in placement. Of course during that process I'm not really concerned with the overall image, just applying edits to exactly the right locations so that I don't for instance get a halo around an object from less than perfect masking. For some parts of the overall process of editing an image, the image matters, for other parts of the workflow it is the pixels at matter, it's just a case of being able to keep the two phases in the correct order.