When I gaming, I usually turn fan speed of my Graphic and CPU to maximum possible speeds. It doesn't matter what is the temperature or load, I just do it always before starting any games. The question is, does it have any negatives to have fans at full speeds even when it may not be needed (should I prefer auto speed setting)?
Are there any negatives for having fan speeds at maximum
Running your fan at maximum speed does a few things.
Make sure that the air that is being moved can safely move to an area where it can become hot without a problem. For example, outside of your computer case.
This is more a personal thing. If the noise is not distracting or discomfortable, its not an issue.
The faster the fan turns, the more resistence it has to endure on both the ballbaring and the motor behind it. Although a fan is tested for a prolonged duration at maximum speed, the manufacturer expects the fan to not always be on nor being at maximum speed. So it will wear down the fan. It does depend on how often you buy a new pc, as a manufacturer usually has like 5 years of life expectency in mind, which can become like 4 or 3 years if you constantly run it at full.
The more air is being moved, the more dust will collect on the fan. If dust is caught in the ballbaring it will slowly trash your fan. So you should clean your fan every 6 months to counteract this to increase the life of your fan.
Although the power requirements for a fan are very low, due to running the fan at the highest speed, it will costs you more electricity, thus the bill will get higher.
Now while that's said, the question remains, why would you want to always set it on full anyway. Since most fans are BIOS driven nowadays, it doesn't take much to automatically make the fan spin up when there's more heat. The only sensible reason to actually set it on max all the time, is if you have a serious heat issue and this is a last resort.
I would therefor suggest to set it to auto and use it unless you really have no other choice. But in those cases, I would look into getting more fans to move the hot air out of the case better.