I agree. Riding with too little air in tyres is worse than too much as you will get punctures, the tyre wears and the ride is soggy.

The reason manufacturers post advice on how many psi to run is because the tyre pressure pushes the rim walls apart, and once the rim walls are non-vertical, hitting something could make them bend, resulting in tyre failure. However, manufacturers test these things under extreme conditions, including a margin of error for over-inflation, force of a heavy rider or riders hitting large lumps and a well worn rim. In my opinion this means that if you avoid these conditions, putting a bit more air in the tyre is unlikely to cause a failure.

The other point to note is that the tyre height has as much influence on the force on the rim as anything else. Reason for this is that if you calculate hoop stress (imagine slicing the wheel in half down the centre line of the tyre, force on the rim = 1/2 pressure x area, where area is the height from the rim well to the inside of the tyre x the rim circumference. 1/2 is included because the tyre itself bears half the stress). This means that running a lower profile (i.e. normal racing tyre) than a large balloon tyre at the same pressure will mean less stress, and is therefore safer.

I (and obviously a lot of other people too) used to run 150psi on my continental clinchers (18 or 20mm, so practically rubber bands) in the days before rim manufacturers posted safety advice on how many psi to run and never had any problems.