A few things . . . Gruber offers a good rule of thumb, and I'd just follow that.
I used to buy second tier "Gold" Seasonic units as a rule, thinking to discipline my cash outlays. In recent years, I used a Seasonic "G" series 650W -- I think it was "Gold" -- to power a system for which I'd carefully measured peak draws through stress tests. Often, some Seasonic or other units of proven quality will sustain draws higher than their rating. So with 2x GTX-970 SLI, my own estimate came to within 100 to 50W of the rated limit for that model. But in the last three weeks I confirmed that it had slowly been deteriorating. Late last year, I removed one of the gfx cards. More recently, I discovered that it would not complete the restart after the "Shutting Down" screen, and analyzed the BSOD dump files. At first, it looked as though it was the graphics driver, and I probably reinstalled several versions of the NVidia driver. But -- no -- it was the PSU -- in some ways, a relief to me that the motherboard wasn't deteriorating.
That system was set up to go into sleep and hibernate. Another similar system with a 750W Seasonic Gold X-Series (six years old last month) was neither intended nor configured for sleep and hibernate, and ran 24/7 except for maintenance days. It, too, had similar symptoms, in that a sleep->hibernation cycle would not return successfully from hibernate about 50% of the time. Eliminating the sleep-state phase of the configured cycle, return from hibernation was totally reliable. But the unit was six years old.
I've replaced both of those PSUs with Seasonic Prime Titanium 650W units. If there'd been a $50 or $60 difference in price between Gold and Titanium units, I figured it was worth it.
But whether it was a 1000W PSU or a 550W PSU, I wouldn't build a new system without a brand new PSU. And it pays to strategically reduce the power draw of the system while choosing a unit that isn't spec'd too far above the maximum draw you could expect. You should always leave maybe 100W of headroom between maximum draw and the PSU rating, but why pay for more if you're not going to use it?