A few of the non-opengl xscreensavers are heat-generators as well -- e.g., XFlame. Most are not. I looked at /etc/cron*, nothing in there is going to generate much heat. I'm going to file a bug against xscreensaver: it should be split into low-usage and high-usage packages. I expect laptop users as well as overclockers would want to install only the low-usage ones.