Cron Run resulted in Fatal Error Message Using Drupal 6.22 
PHP 5.2.17
PHP memory limit - 150M

Last time I ran Cron was 2 weeks 19 hours ago

I am experiencing no errors with the site but opted to run Cron manually and received the following error message

Fatal error: Allowed memory size of 157286400 bytes exhausted (tried to allocate 7680 bytes) in /home4/xxx/public_html/xxx/includes/theme.inc on line 1078

Is this an error message I need to be concerned about?
If yes, How do I fix it?
And...Is running Cron a necessity, if the site is functioning fine everywhere else.
Whatever cron is doing, you'll need to allow it more memory for it to succeed. You can find out about increasing PHP's memory_limit at http://drupal.org/node/207036

However, it looks like you already have memory_limit set to a high value for a small site.