I wonder about the long-term effects. Turning of nap will increase the heat and work the cpu does. I'm not technician but I think the added heat will shorten the life.
I've used this on my pb to stop the buzzing there but because of its small enclosure, and tiny fan. I've given that up and lived with it.
You don´t have to worry about any long-term effects. I have checked out that there is no higher temperature of the CPUs if you disable "nap". The fans spin up a little as they will do every time you use the computer with any process. A computers components are supposed to work if you use it anyway. It would be the same to worry about letting a program run - and thats exactly wherefore those computers are built!
All those modern 64-bitters have a function like this - but it´s not more than some kind of smartness, noone needs really - maybe it makes sense if you dont turn out your computer ever - but in this case I would suggest you just to enable napping again before leaving.
The new 64-bit Athlons have this function either - called cool ´n quiet there - and causes a lot of problems with all programs that once take a sample of the CPU-speed to generate a stable timecode... If the program does not fill out the CPUs capacies, it will slow down by its own decision - and the timecode then generated is - well, lets say: very unusable... Only solution: disabling this "feature"
At the G5, I personally think, the heat of the CPUs is not a main-factor of life-shortening at all - mine has stable temperatures around 20° C at most times - if you think about the P4s and Athlons with their temperatures of 59° to 64° C when running normal - this would make me more worrying about the lifetime.
The only critical temperature at the G5 seems to be the drive-bay if you are using two disks around more than 200Gig capacity - this should be watched, more than the CPUs!