#1
I keep tuning down to drop c every now and then. Do i need to adjust my truss rod each time? Or it doesn't affect it that much?
#2
Only reason you'd need to adjust the truss rod is if there's any bowing on the neck. You can adjust it to make the strings fit tighter if you'd like (while you're in drop C of course) but when you tune back up make sure to put it back to normal as to avoid warping of the neck due to the string tension being too tight
"We programmed in death...A thousand needles lie here to inject their lies."
#3
^ String tension will never warp the guitar's neck. That's exactly what the truss rod is there to prevent.

To answer the TS' question... No, you do not need to adjust it. If you were keeping a guitar in a certain tuning permanently then it would be good to get the action and neck bow(truss rod adjustment) for that specific tuning. Otherwise, there's just no point.
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