Cron jobs will always run as the user whos crontab they are in, or root if they are in /etc/cron.daily, etc. If you are concerned about them eating CPU cycles, prefix the commands with nice, which basically assigns the process the lowest possible priority. See man nice for details.
FWIW, there are many processes that run as "nobody." Apache is probably the most common one. Under a default 2.0.x install, all Apache processes run as user nobody, group nobody. Its nothing to worry about from a security perspective.