Question for D tuning (full step down on all strings)

I would say, if you are going to do this, do it right. If you're just going to mess around, don't go crazy. But if you want to really do it, dedicate a guitar strictly to this down tuning and you won't regret it.
 
I'm permanently tuned to a C below standard E as that is how I play and the only tuning I know. There are quite a few things to watch for me, but similar rules apply even when going down a whole step.
 
I actually use 9-42 on my guitars, but it works because the guitar is literally set up for that tuning in all aspects. The action, the truss rod, the intonation, the trem bar...and even the saddles are slightly modified, as I put little washers under them to raise or lower individual height along with custom saddles that are just 1/s and 2's. 3's are too high.
 
That said, the heavier gauge strings will always be a good choice. But just remember there are pros and cons to all of this. One of the biggest things to watch for other than string buzz, is dual tonality. What this means is, you can literally pick up two tones on a note due to the pickup magnet grabbing the string two times because of the lack of tension as well as being too close to the pup. So you will need to adjust your pup height in most cases.
 
The good thing about lighter gauge strings is, you won't have to deal with this because the string thickness isn't there. The con here, the strings are so loose, you can bend nearly an octave. LOL! 10's or 11's are good choices in my opinion, but you may be better off with one of those custom sets that give you thicker bottoms and normal tops for easier lead playing IF you play lead at all.
 
The good thing is, because you will be tuned a whole step down, the extra string thickness won't be much of a change for you over being tuned 440 using lighter gauge strings. Being tuned lower will compensate a bit. But the thicker strings will definitely make the tone thicker too. But that said, you can use just about any string gauge once you set the guitar up for that gauge. No one believes I use 9's on a C tuned guitar until they try it.
 
Also, a little string buzz is good. There is a difference between a little buzz adding to the crunch of the tone vs. fretting out buzz. That, you do not want. But mine buzz ever so slightly, but I can only hear it when the guitar is not plugged in. You know, if I noodle around with it and I'm not in an amp or pre-amp, there's a slight buzz. But it's a good buzz that actually adds to the sound.
 
What I'd suggest is this....
 
Choose the strings you feel the most comfortable with
 
Set action a little high after you are all tuned up and staying in tune
 
Put some heat on the truss rod....meaning, tighten it so that the action appears to come down lower. From here, experiment with lowering action if need be vs. tightening/loosening the truss. You need to find a happy medium here and also remember that all the standard truss adjustment stuff we are taught for 440 standard tuning will no longer matter for you. The truss rod is adjusted for feel now...and you may have to put a considerable amount of heat/tightness on it to even things out.
 
I always make my action a little higher, tighten the truss until I buzz a little, and see how I like it. Normally, I will want the action a little lower. So I'll drop the bridge a bit and then loosen the truss a little until I have that good combination of tight feel from the truss, and good action that just feels right. You'll know when you have it dialed in. Good luck!
 
-Danny