Yeah, I do mean to take it completely off, go vertical and along it to drain. Here are the reasons:
1) That water has chlorine/bromine/etc in it. You don't that near your protective layering system around the foam core. It breaks it down.
2) Like Doc mentioned, typically this practice includes when people are shocking. Oh my! Don't do that!
3) Doesn't happen in a lot of cases, but we have seen cases where the spa owner keeps the temp cranked up to 104+ which allows the bottom material to soften and be prone to expanding. If there is trapped water in there that hasn't made it out the weep holes, gravity starts to take its toll. Like I said, it doesn't happen all the time, but no one wants the bottom side of their cover riding down in the water.
Sorry I wasn't clear before.
