I was told by a guitar tech when I was having my Strat serviced that you shouldn't leave strings off your guitar for extended periods. He said once a guitar is said it needs the resistance or pull of the strings and the neck can warp without it. Now... that's what I was told but I never had a knowledgeable source to verify that with. It may be worth checking out.
I was looking into your situation and I found the below info and similar info on other sites. If there are people in the know in PB land could you verify this information is correct?
"Strings put appox 120lbs of pull on the neck. The truss is the counterbalance to that pull. If no strings are present, the neck can backbow depending on the neck thickness. (Thin necks rely on the truss more than a thicker neck)
When you say have you strings off for awhile, if its a few days, its unlikely any perminant damage will occur. If you're talking months or years, the truss should be loosened to avoid backbow. unless its a piece of junk you dont care about, you always want to avoid extreme changes in truss pressure like that. Once moisture sets into the wood, just putting strings may not allow for enough forward bow to occur and heat may be nessasary.
I do alot of guitar restorations. Getting instruments left in a closet half strung is a pretty common thing. On an acoustic, Loostening the strings about 1/2 way helps prevent the top from collapsing over time yet still leaves some cpounterbalance to the truss. On an electric is always neck issues. It can do anything from make the neck twist to causing frets to pop out. Frets are held in place by the forward relief of a neck. Having reverse bow the frets are free to pop out. Storage with say only the heavy strings on (which is really common because beople break the thin strings) can cause the neck to twist. Neck twist is the absolute worst of all evils and is often a lost cause to correct. Best you can do is remove all frets and level the fretboard, but thats only good if the problem is mild.
So my best advice is, dont remove the strings unless you're changing them and replace strings one at a time if you want to maintain your setup. If you're going to store it for long periods without strings, loosen the truss."