If you use soap (or WD40 etc) on the tyre bead when fitting, give this time to disappear. I've seen bikes ridden quickly straight after a tyre change and guess what - they spin on the rim. Definitely leave the valve nut off or fit a mousse (or tyre balls) if you're really serious

Also, my experience of security bolts is that they are OK for mainly off road, but on road the wheel is so out of balance that it makes the whole bike shake. Personally I agree with the pinch flat theory and would run a slightly higher pressure and no security bolt.. Bit of a compromise either way I guess..