Air Pressure in your tires suspends your expensive auto from the pavement. It's important to make sure you're running just the right amount for the conditions you drive in. Many BMWs, Porsches, Audis and other vehicles have a Tire Pressure Monitoring System (TPMS) which warns you of 25% fluctuation in pressure. That is quite a swing in pressure (~9.5psi swing when tires at 38 psi). Your car may have different handling characteristics with a 5psi change.
Even if you have this system, it is wise to check the pressure monthly or as your driving conditions change (ex: loading up for final weekend away with the family vs commuting to work each day)
The rule of thumb is for every 10° Fahrenheit change in air temperature, tire pressures will change about 2%. This means that standard-pressure tires will change by about 1 psi. We have all experienced 50 degree swings in temperature here in CO in a day. Parked in the sun all day, that effects pressure also.
With all these factors that effect your vehicle's handling, it's important to be aware of your tire pressure. Check it "cold", before any long distance driving and try to do so about the same time of day so you have consistent.
Enjoy the end of the summer!