For me I have a growing LTO-6 archive (approaching thousand of LTO tapes) stored in dust proof and water safe case (Pelican) for 4 years now. Recently I’ve been restoring them and all OK. I even cover my external LTO drive in a dust proof container when not in use. This helped tremendously in reducing the # of cleaning cycle. Over time dust accumulated in the drive will make it very hard to read or write. I recently experienced this with one of the HP LTO-6 ext drive. The backup would be all OK but during the verification process, it would take 3-5x longer than normal. That’s when you know something is not right. After the cleaning tape is used, it’s back to almost new condition.
Storing the tape vertically sounds like back to the video tape era (before 2005). This makes sense because if the LTO media are stored horizontally, the 1/2″ tape may get wrinkled on the bottom edge over time.
Rewinding them yearly is a bit too much work, especially when you have hundreds of them. If the storage condition is relatively good (40-50% relative humidity and always 65-70 deg F), every 3-4 years is fine. This retensioning process is to properly redistribute the tape and avoid it being cinched and tape backing deformation. It also cuts any chances of fungus or mold growing in these LTO media as well.
I also see corrosion being a factor. If the media are stored near the ocean or where there’s high content of salt water in the air, the tape will be affected. You can see this affect by bringing your camera gear to the coastal area. After couple of months, the screws are corroded. What’s bad is that inside the lens elements are corroded as well.
Typically I’d also give it couple days for the tape to acclimate to the current environment before restoring. If the tapes are taken off an archival facility or room to the server room with different climates, giving it a few days before restoring is good to do.