Theorycrafting about how good your PSU is one thing, but real-world performance is always below what is theoretically perfect. In your case, something in the 650W range would you give you headroom for more overclocking and also give you an upgrade path in the future.
It's generally regarded as very bad to run a PSU near it's rated capacity for any length of time, you ideally want to stay well below 90% of rated capacity to be sure. Most PSUs have efficiency curves which are classic bell shapes lowest at the minimum and maximum load, if you are loading the PSU at an average of 50-60% you are typically where the PSU is most efficient. 80 PLUS certification is actually done at 20%, 50%, and 80% loads, if you are exceeding 80% or below 20% of rated capacity you are not at the point of the load curve where the PSU is running most efficiently.
Assuming you are running at maximum capacity, the chances of sudden catastrophic failure of a PSU increase dramatically. The PSU is generating a lot of heat which makes the fan run harder. The components in the PSU will be at or even beyond rated capability. Most decent PSUs will gracefully experience sudden failure by just magic-smoking themselves, but if you are unlucky it will destroy your motherboard, CPU, and random components connected to it when it fries. If you don't like the risk of sudden system meltdown, it is not advisable to run any PSU at maximum capacity for any length of time.
Like all things made by man, PSUs tend to degrade over their lifetimes. After a year of average use, a PSU is around 80% of it's original rated capacity just from normal degradation of components. This is not unlike how the Lithium-Ion battery in your cell phone, laptop computer, or whatever degrades over time. If you've owned a cell phone for more than a year or so you know that the battery eventually cannot hold a charge any more after gradual degradation of battery life. If you build a box which is marginally within the PSU's ability to feed, in a year you might be running the PSU beyond what it can handle. You don't really want that to happen.
There are many reasons to buy more PSU than you think you will need, and considering how cheap your average decent quality PSU is these days, it would be quite frankly stupid not to invest in a slightly more expensive, slightly more powerful PSU than you think you will need. The PSU is what makes the whole rest of the computer work, after all.