DC-DC step down 12V to 5V source versus straight 5V source
Assuming a generous 90 percent efficiency, that's still 45 Watts in for 40 Watts out, so a 12V supply just to bring it down to 5V would not be efficient compared to stepping up at 80 percent efficiency for 1.5W in 1.2W out
 5 Watts vs 0.3W efficiency penalty.  Even if you boost to say 14V and use a LDO for a cleaner signal for your OP amps, your still looking at 10 times worse power loss with a 12V supply.

Cost is negligible for supplies for 12V 5A or 5V 8 to 10A, but a 45 watt step down converter will cost more than a 1 watt step up converter.

So in my opinion, it makes more sense to go with your natural voltage need.