They recommend more if you'll be overclocking. Seriously, though, I don't overclock my i7-960 or my 580s and my GPUs reach full load at times while gaming. If you aren't running old games or everything in 1024x768, you won't have to OC anything to get peak performance out of your 570s. When you OC, you always have some unknowns with the requirements. I don't do it, but people say you could require an additional 100W+.
The most my system has required while gaming with the GPUs pegged was just around 670W with no OC.
Your system would require around 20W less per card (-40W) and 25W less for the processor with no OC putting you right around 605W. Add OC to your components and you'll drive the power requirement easily to the edge of what the 750W can provide.
My recommendation if you're going to OC the video and CPU would be to bump it up again to a Corsair 850tx:
It's only $20 more than the 750TX, but will give you some wiggle room to OC even when you have two 570s. All in all the 850TX will give you around 200W of overhead (once you have 2 570s) for OC'ing things.