I was wondering how many folks out there let their editing machines adjust monitor brightness based on ambient lighting versus just using a static brightness setting.
A bad idea and practice to allow auto compensation for ambient lighting IF your intention is to print and you require accurate soft proofing.
A good idea and practice is to keep a constant ambient light level in your editing environment that is comfortable for you and adjust the monitor luminence level to allow for a print to screen match.
If you cannot keep consistent ambient light conditions and assuming you have tried those new fangked curtain things it is still poor practice to let automated brightness take over.
Should you not wish to print there is less pressing need for accurate calibration although it is still a good idea to calibrate to a given standard and hope that others that may view your images have at least attempted some form of voodoo to make their screens usable.
Should you wish or need to have an accurate view of your image data then monitors should be calibrated regularly. Monitors will shift over time luminence and colour. Calibrating to your required standards will adjust the resulting profile to take into account these shifts and therefore present your data correctly. You cannot do this by relying on eye alone: by the time you notice there will have been a fairly large change