The typical reason why rack bolts fail is from fatigue. A rack flexes with pedal strokes and road surface shocks. If the rack isn't well braced (and many would be surprised at the number of racks with no lateral triangulation) this flex will be worse. If the rack bolts are doing double duty, like fender brace attachment too, then again the bolt's flexing will be worse. If the bolt is allowed to become loose the flex will be greater. And in time the paper clip breaks, so to speak. Increasing the bolt's diameter by about 20% sure extends the life span of poorly dealt with bolts. Note that last bit "poorly dealt with". because for most people's touring a well engineered rack using M5 bolts that are kept tight and don't do any other job are quite strong enough.
it's when the tour is over lot's of unimproved/unpaved roads and the pack loads are really high (think 75+ lbs) and the consequence of a broken rack/bolt is possible unhealthy (think third/forth world countries, deserts and such) that going to a M6 bolt is really a smart idea.
In my shop experience (and of course my personal touring which last time had a bike and load weight of over 110 lbs) the number of broken rack bolts has been less then the number of eyes damaged by loose bolts, by over tightening or by using a miss matched bolt/screw (like dry wall screws
). I even have heard of eyelets breaking off the drop outs from too much flex and stress. On my first Scotland tour the Eclipse front low rider rack strut broke, after it had seen thousands of loaded miles over the 15+ years of it's being in use. But bolt breakage is not too common IME.
Still were I to make a third world touring bike I would strongly consider going with M6 bolts and their bigger eyes and bosses. But those dreams are in my past so M5s are fine for me. Andy (who for his touring bikes followed the one bolt does one job policy)