When changing the priority of a process Windows 7 warns:
Changing the priority of certain processes could cause system instability.
Should I care? what does it even mean? what are the implications of "system instability"?
No, it's just a warning. You probably aren't going to cause any problems by turning the priority of a process up or down a notch. And there won't be any irreversible damage; anything you do mess up can be fixed by a restart. Just make sure to save your work first before experimenting!
There are a couple of common things that can go wrong (and thus that you should watch out for):
In my experience, assuming a decently fast and stable machine, as long as you stay away from the "Real Time" option, you will be fine. Turning non-system process down in priority level is even less likely to cause harm.