There are only 2 signals for an impending valve adjustment:

1. you don't know when last adjustment was performed

2. last adjustment was more than 15K miles ago

BTW, the adjustment won't give awesome performance boost anymore than disabling EGR will. But, much like the disabling of the EGR, the results of consistent valve adjustments are cummulative and will give you an engine that lasts longer and is stronger than it would have been without the service. So, if your car went strong for 100K or 300K miles without valve adjustments, it doesn't mean that the rest of us are chasing our tails performing the regular service. It just means that your car would have gone longer and stronger with the service.