For CPU clients running on a desktop system, I'd say the danger is close to zero, assuming you don't have some marginal component in there that is about to fail anyway.
For GPU clients, it's gonna depend. Pushing a GPU hard 24/7 won't be an issue if it has a quality cooler on it... but I've seen an awful lot of video cards with marginal coolers.
I would hesitate to fold 24x7 on a laptop unless you've got some extra air blowing on it to help keep the temps down.Edit
: To give you some real-world anecdotal evidence for desktop systems... I've run folding on systems for years without ill effect. This has included my primary desktop systems, home servers, and cobbled together diskless "crate farm" systems
. The record holder is probably my home file server box (an Athlon 800), which folded 24x7 for nearly 5 years (the system still runs fine, I just don't fold on it any more). My current web server (an Athlon XP 1800+) is also approaching 4 years of 24x7 folding. My current "heavy hitters" are a trio of Athlon X2s (and a single Phenom X4); even with stock cooling, the CPUs do not get very hot.
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