I've just had my tyres filled with Nitrogen, after buying two new tyres from Mr Tyres, the molecules are bigger than mixed air, so it doesn't seep through to the alloy and cause corrosion (there being only pure Nitrogen, in the tyre, no water or other contaminants. And, there's no need to check the pressures for two years! Mind you, that's more frequent than I normally check tyre pressures.
For the price of £1 a tyre you can drive into Mr Tyres and have them checked and refilled as necessary, additionally, if you have to use mixed air, whilst out and about, they will refill with Nitrogen on your next visit
I've had the 32 psi rule, for tyres, all my car ownership years, if in doubt use 32 psi, all the tyres on the LS400 are at 32 psi, funny that