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Best to keep strings on it because the soundpost is held in place only by string tension. Granted, this isn't permanent damage as a luthier can reset the soundpost quickly, but it's better to leave the strings on and save yourself a trip to the shop. (Also, playing a violin with a badly out-of-place soundpost could lead to cracks or other structural damage under some circumstances.)
Yeah - equally - I agree for the reasons above by others.
You MAY wish to "relax" the string tension slightly - just a tad -(but don't remove them) on the acoustics (oh - and relax the tension on your bows!).
For the EV, I doubt if it matters much, I can't see anything untoward happening - well, a peg may slip, but that's a minor inconvenience...
Even a SP fall on an acoustic is nothing drastic, really, easily fixed (first time can be worrying, but it's not difficult) - what MAY be more significant is (depending on the temperature profile and variation when you're away, and whether the pegs slip or not) you may, possibly end up over-stressing the strings slightly, or, the structure of the violin body. Sure - there's probably not MUCH in that - I mean I regularly play one of my fiddles tuned a full tone up across all strings (nah, don't ask!), and it has never suffered, over 5 years from that - but - worth thinking about.... (that is - relaxing the string tension, but not completely removing them)
I seriously recommend not copying my mistakes. D'oh -
Please make your own, different mistakes, and help us all learn :-)
I remember a lady here that went on a trip overseas, taking her violin along because she needed to practice while there. Her sound post fell and she didn't know what to do. The advice given here was that she should wait until she got home to have it set by a luthier. My thinking is that everyone should know how to reset a sound post so that they don't have to live in fear. The tools I used for two years were a plastic straw, a length of thread and occasionally a straightened paper clip.
I have always felt sorry for that lady and wished that I could have helped her.
Relaxing the tension on the e and a strings would be good.