36h is plenty, unless you're on a tandem, or if you and/or your gear is particularly heavy. I tour on 32h (front and rear) and my rims don't go out of true over the course of a 5,000 km tour.
As a few others have mentioned, it's also a lot easier to find replacement parts for 36h, since 40h is uncommon. 32h is generally even easier, but I understand choosing 36h for peace of mind, especially for the rear, especially if you're a big guy.
It's not a bad idea to go with 36h rear, 32h front, but I simply got a good deal on a 32h wheelset, so I went with that.