No. They're built by professionals so they'll be of good quality.
You could email them to ask advice on what specs you should pick for overclocking if you like.
the oc'd pc from PCS would be a lot more stable than other overclocked amateur built machines, mostly because PCS will have done professional research into the best settings and hardware compatibility for overclocking. Overclocking is always a risk but with buying a pc from PCS you will also get the added peace of mind of a warranty, which is always nice.