Of course, if you just drop a fistfull in the sound hole, you'll always have some handy - if you can get one out.

The spring loaded holders I've seen all have the "foam" manager tape pre-applied, complete with 3M markings on the peel-off. It holds quite securely, but like any contaminant, I'm sure it will leave a mark if you remove it - at least if it's been there for a while. Those I know who've put them on (self included) usually just leave them there long after they've forgotten there's a bunch of picks there. (If you use one out of the holder, you have to find another one to replace it - no?)

John