The recommended tyre pressures are set when the tyres are at ambient temperature (cold), the tyres have not been used for at least 3hrs.
Once one starts driving the tyre temperature will increase due to friction and flexing. This rise in temperature also increases the tyre pressure, which as it increases, reduces the amount of tyre flex and in effect stops the rise in tyre temperature and subsequent increase in pressure. (which incidentally is why under-inflated tyres are dangerous, as the low pressure allows prolonged flexing and a dangerous increase in pressure)
The recommended 'cold' pressures are calculated to give the designed running or 'hot' pressure. Ambient temperature changes will obviously change this starting point and tyres should be adjusted from winter to summer and back again. Generally in the UK we don't experience huge variations on temperature within the seasons (there are exceptions...) so I tend to allow small variations in 'cold' pressures as the running pressure always evens out the same.
Obviously one must always check wear patterns and adjust pressures accordingly.