Apple got a lot of bad PR for the build quality of the G5s. But most of the one's that had those problems have already died.What if this happens to my G5! I hope not, fingers crossed it will last for a bit more.
My PowerMac G5 lasted eight years and three months of continuous use and being left on 24 hours a day seven days a week (at full power I might add, I never let it sleep). That was through power failures and heavy use which kept the CPU very busy.
If your G5 is alive right now I wouldn't worry. You have one of the few (like I did) that were built right.