While genrally versions 3.60 or 3.65 are recommended because of their coldboot abilities, if you found a really good deal for a 3.67 or 3.68 vita, just go for it. Running h-encore is fully offline, and it really takes only 5-10 seconds to launch it after a reboot, if you leave the device in sleep mode there is no need to run h-encore after waking the device.
So, not having coldboot ability is only a very minor inconvenience. There is no need to desperately search for a vita =< 3.65 or pay an extra premium for a =< 3.65 model.