I would caution against using high-strength bolts to attach things such as racks. Even though the weight on a rack may be high, I would suggest that most ordinary bolts have lasted very well for many touring riders, and certainly in my own case.
Bolts in those situations usually break because they haven't been torqued properly. However, and here is the point of my post, if you use high-strength bolts and there is a "need" for a breakage, it will be your frame that breaks, not the bolt.
elcruxio, just what do you intend to carry that would create so much weight on a rack that is has you so concerned you have to use M6 bolts and even double-bolt? As I alluded to above, your rack and/or frame are going to fail before the bolts, and I know that I would prefer the bolt to fail first.
Stainless M5 are fine if you are concerned about rust. So are zinc-plated bolts.
I would suggest further that you invest in some nickel based thread prep; it's black and not as "clean" as grease (the stuff I have is black), but it is meant to prevent corrosion between threads of different metals and so far for me, so good.
However, if bolts continue to undo, Loctite 243 (definitely not the red variety) is a solution. But if proper lockwashers and flat washers have been used, Loctite shouldn't really be needed.