The danger in leaving them full is that, if they are left in a hot environment (car / trunk on a hot day, or leave your bag in a room with sun hitting it through the window) the gas pressure will increase, in some cases until the magazine deforms. I've had 2 magazines do this, in my case deforming at the bottom end where metal pins try to hold the end 'cap' or 'door' against the rubber bottom seal. Repairing a magazine from this can be done in some cases, but leaves the metal weakened.
The reason some people say the pressure is similar no matter how full, is that propane is usually stored in the magazine compressed in *liquid* form (that's why you fill with the propane bottle upside-down, to get the liquid at the bottom. As long as there is liquid, that liquid will vaporize to equal out the pressure, so the pressure inside the magazine is *almost* the same right up until the liquid runs out, then as the gas escapes the pressure goes down quickly.
While this evenness of pressure makes for consistent shots (hence why propane / green gas is preferable to, say, CO2 which doesn't liquify at the pressures we use) it means you have to *almost* empty your magazine if you want to guarantee not to have deformed magazines.
So, to do this, you would completely empty your magazine (slowly, maybe by firing the gun. Opening the release valve is only bad because it cools the magazine *really* fast which isn't great for the metal) then put some green gas / propane in but this time DON'T turn the bottle upside-down, do it right-side-up, allowing just vapor to enter the magazine. You could even fire it once or twice after that to ensure there is only partial pressure in it.
Of course, you can protect your magazines by controlling their environment - keep them in a cool basement for example, and they should be just fine there too. That's probably what most of us do, since your house is probably at a pretty constant temperature all the time.
Hope that helps!