I have several guitars in cases that I really don't play. Is it necessary, or a good idea, to loosen the strings, or just keep them in tune?

My advise would be to keep them in tune. That is the way they were designed to be kept.
there may be other ideas. Or Evan better yet, since you don't play them anyway. Give Them To Me and I'll play them for you.
----Danny,

Keep them in tune.

I have a guitar that I literally left under the bed (it did travel from house to house to house) for 30 years. It's perfectly fine and set up quite well when I pulled it out and started playing again.

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2002 mim Telecaster
Jimmie Vaughan Stratocaster
Mexican HSS Stratocaster
6 non-fender guitars
2 kids (not for sale)
1 wife (uh....no comment)

If you don't take Danny up on his offer, I could play them for you.

They should be fine as is.

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It wasn't Willy-Nilly, it was at crows.

I enjoy having them and always will, but I would really enjoy listening to any of you play them. I can't begin to do them justice. Thanks for the replies.

You'd really love to hear me playing them, just send them over and I'll send you a tape. Hee Hee Hee.

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'65 Strat,65 Mustang,65 Jaguar,4 more Strats,3 vintage Vox guitars,5 Vox amps,'69 Bassman with a '68 2-15 Bassman cab,36 guitars total-15asst'd amps total,2 vintage '60s Hammond organs & a myriad of effects-with a few rare vintage ones.

30 years under the bed with it strung to pitch????

If I know I'm not going to touch the guitar for a long time - I'll tune it down evenly about a step or two. Then I'll fold over a small rag and place it between the strings and the pickup pole pieces. Then I'll store it standing up (case). That has always worked for me.

If I leave them strung to pitch I take them out at least every three or four months and pull the strings and then tune them and play them a little and check intonation..

I would think that if you strung the guitar to pitch and intonate it--- and put it away for a long time and then come back and check the intonation..... It makes sense that if the intonation is out then the neck has moved slightly.