Adjusting the relief on a guitar or bass neck is generally quite simple and safe to do. It’s something we think every guitar or bass player should at least be familiar with doing, in case of an “emergency” when a repairman or luthier isn’t around. With our own guitars, we like to check out the instruments’ neck relief, as well as the setup and intonation at least a couple of times a year (spring and fall, when the temperature and humidity change in many parts of the country, are prime neck adjustment times). Some players give their instrument(s) a relief/setup/intonation check every time they change strings — not a bad policy.
Having said that, it is possible to damage an instrument’s truss rod or neck by taking things too far. If you’re unsure of how to proceed with neck adjustment, we recommend asking a qualified tech to show you how the process works.
By all means, if you’re uncomfortable making adjustments on your instrument, take it to a professional. And, if you can’t seem to get the adjustment right or there doesn’t seem to be enough adjustment range to get the relief you want, definitely take the guitar or bass to a qualified tech!
Ernie Ball and Fender both make convenient tool kits that cover everything you need for basic maintenance of your instruments. Taylor also offers specific truss rod wrenches for adjusting their acoustic guitars.