I'm a similar weight and have always run wheels with alloy nipples (build by dave thomas at speeddream.com). I rarely have had to true the wheels but when i have, i've had no problem with the alloy nipples.
Why do you recommend brass nipples for someone ~ 190 lbs.? I'm considering starting to build wheels (one of the few things i don't do myself) - and I'd appreciate a pragmatic view on this issue.
My understanding of this is that brass nipples are generally more durable. As the wheel rotates, the components of the wheel are constantly being loaded and unloaded. This is what mainly stresses the rim, nipples and spokes. The greater the load (bigger rider, aggressive riding) the more wear and tear you get. IME, the wear is usually first noticable at the contact points of the different components of the wheel (nipples/rim and spokes/hub). Therefore, brass nipples will generally outlast alloy ones- particularly under a bigger rider. That's not to say that big guys can't use alloy nips, just that brass will likely save you some trouble if you're planning on riding these wheels for several years.
On the spoke question, I have noticed that double butted spokes are just as durable as straight gauge. Almost all spoke failure occurs near the hub or the nipple. I can't really speculate on the comment regarding stiffness of one over the other, although it makes some sense that straight ones would be stiffer. I don't think that because they're stiffer they would fail more easily, but I'm just trying to use my imagination on this one.
Honestly, I'm not a particularly experienced wheelbuilder. I have built some wheels and been around some pretty opinionated (and well respected) wheelbuilders but I'm no expert. I've definitely repaired a lot of wheels though and I've seen what you big fellas can do to a light weight racing wheel... There are lots of good resources for this on the web, and other forums here on RBR that will get you some good advice. This is hardly a cyclocross specific discussion, any well built road wheel will put up with the rigors of cross.