If you occasionally de-tune your bottom string to a D, you can probably skip this one. You’ll be ok.
However, if you regularly (or always) de-tune your guitar, you might want to give some thought to your setup.
Different tunings mean different string tensions. This, in turn, means different settings for relief, action, and intonation. String tension affects all of these variables and you should take this into account.
Before doing anything, consider what tuning your guitar lives in most of the time.
If you only drop for one song in a set, it probably doesn’t make sense to set up your guitar for that tuning.
However, it’s worth mentioning that if that song is tuned very differently, some of these setup issues may make trouble. If this is the case, you’ll have to make some hard calls. Either live with it or, maybe, dedicate a second guitar to your second tuning.
If your guitar always lives in your dropped tuning, great. Set up for that. Tune to A (or whatever you crazy youngsters use these days) and setup your guitar in this tuning as you would for standard tuning.
Heaps of different tunings
Things get more complicated if you are changing tunings every song or two. Depending on how ‘sensitive’ your guitar is to tuning changes, you may have to compromise (see considerations below). Some guitars are rock solid and the neck won’t move no matter what happens and some will shift a little with more or less string tension. You’ll have to get a feel for your instrument and experiment to figure out the best ‘middle ground’ for all tunings.
Setting up for dropped tunings
You’ll need to consider relief, action, and intonation. Dropping your tuning is not unlike changing to a lighter string gauge and the issues there are similar.
Lower pitch means less tension. This means the strings don’t pull as much and your truss rod might be over-correcting neck relief (bow). If you drop tune, check relief and slacken off the truss rod if you find the neck’s become too straight or has back-bowed.
Staying on the subject of relief, the reduced string tension means floppier strings and a little extra relief or bow might be a good idea. Big chugging chords may benefit from a bit more relief than a guitar in standard tuning. Experiment and see how cleanly things play.
Depending on your tolerance for buzz, that lower tension might warrant a higher action. Again, the floppiness of the slacker strings gives more ‘wobble’ and therefore more potential for some buzz. Consider raising action if needed.
Your intonation depends on string tension, action, and relief and will certainly need to be checked in your new tuning.
In a correctly balanced tremolo system, your string tension is matched by the spring tension around the back. Changing string tension will need a compensatory change to the spring tension. Depending on how low you tune, you might have to remove a spring too.
A note on strings
Lower tuning pitch means less tension.
However, heavier strings means more tension for the same pitch.
Therefore, it’s VERY useful to consider installing heavier strings if you’re tuning low. VERY useful. If you’re planning on dropping to Ab with a set of .009s, you’re in for a tricky time of it.
Go heavy. Don’t be afraid of heavy strings. As you drop tunings, heavier strings are much more manageable to play and are more stable than their lighter counterparts would be.
Highly recommended: Go heavy. Fear not the 12 and 13 gauges. They are the de-tuner’s friend.
Of course, if you get heavier strings, you should also remember that your nut slots will likely need some work too.
Still, it’s the right thing and will make all of these setup issues/steps easier.