Recommended pressure varies a lot by car and tire.
If the sidewall is rated 51, I trust that the company's engineers (and even more, lawyers) are willing to put their reputation on that pressure.
Above that and you're on your own. I'm ok with that. People do things all day every day that are outside the recommended norm.
The real danger I see is with low pressure. There is where you have blowout risk. See: Ford Explorer / Firestone. Heavy vehicle with low specified pressure (26 psi!). Higher pressure reduces sidewall flexing, which reduces sidewall heating. Lower pressure means they flex more, heat up more, and are more prone to failure.
