Yes, I like to see about 100w MORE than what the system is probably going to use under a full load. If you can estimate that the system will use about 350-450w, then a 550w unit is a good choice. If you have plans to overclock anything, then you'd want to add another 100w and a 650w model would be a good choice. It really depends on the hardware and the usage, but in general using the recommendations at the following link, then adding another 100w to those recommendations IF you plan to overclock the CPU and graphics card, are a good idea.
http://www.realhardtechx.com/index_archivos/Page362.htm
If you have no plans to manually overclock anything to any significant degree, then a good unit based on those recommendations is a fine choice for practically any configuration around the graphics card the recommendation is made for there.
A person can pretty easily estimate the actual needs of the system themselves if they wish to though.
For example, if you are running an i7-8700 which Intel says is a 65w part, but we KNOW that it can use up to ~117w under a full load due to review data. So, 117w max for the CPU.
Then, you have a GTX 1080, which on average will use UP TO 75w for the slot, and it has only a single 8 pin for most models BUT some models have up to two 8 pins, so we'll go with the dual 8 pin selection which means 150w maximum draw per 8 pin for a total of 375w potentially just for the card.
Add to that the 117w of the CPU and you have 492w theoretical maximum power draw, so far. The rest of the system including the motherboard, USB ports, SATA power cables for storage devices, fans, lighting, etc. might well run up into the 100w range. Likely no where near that much, more like maybe 50w, but we'll say 75w just to split the difference and play it safe. If you had five or more storage drives, a bunch of external USB devices, 7 case fans, the CPU cooler, four sticks of RAM, and an extensive number of LED light strips, we might opt towards the higher number but that isn't the common system PLUS we're going to pad the number a bit towards the end anyhow just to be safe.
So now, we have 117w for the CPU, 375w for the graphics card, 75w for the rest of the system, for a total of 567w. Let's add another 100w to that in order to ensure that there is plenty of headroom to accomodate any spikes in power consumption, which definitely happen, and to help ensure that when the system is running at full load we are not exceeding about 80% of the power supplies maximum capacity, so that it runs in the "Goldilocks zone" where it has the best ripple, voltage regulation and efficiency performance. Generally, about 40-80% of the units capacity is where you want to land when running it under a load and for me, 60-80% of capacity is even better because a lot of units are much less efficient when running at a significantly low percentage of the units overall capability. In truth though, I'm a lot more concerned with the unit not exceeding 80% of the units maximum capacity so that the numbers for ripple and voltage regulation stay in the units most promising range.
Nobody wants to see high ripple or poor voltage regulation, which are unlikely anyhow if it's a very good unit, but which become more of a concern when you get up to using 80-100% of the units capacity on a mediocre or poor quality unit, but to some degree even on a very good one.
So, enough of that ramble, we are now looking at a total of 667w including the extra 100w we added. Technically, this configuration could probably run fine on a 550w unit, as per the most common recommendations.
For our estimate, even though we end up above the 650w mark by about 17w, we can reasonably assume that a 650w is a really good fit for that system because the 117w we got for the CPU was taken from it's usage while running Prime95 during a torture test, and we are never going to see that kind of usage under real world conditions, so anything from 650w-750w is a good choice. If I was buying a Seasonic Prime Ultra Titanium for that system usage, I'd probably feel great about a 650w unit. If I was looking a somewhat lesser quality Corsair CX model, I'd probably want the 750w unit.
Having some additional overhead in terms of not using a PSU that barely covers the capacity needs has one additional benefit as well. It is going to run much quieter under most conditions, even while under a full load, than it would if you only had about what the system needs or only slightly higher. Not having a noisy PSU is a good thing, both for your sanity and for the life of the PSU and it's fan as well. So, win-win in that regard.