Maribeth - I've also found using any soft, rollable table cover is good for this, with the added benefit that you can't lose a single card. Once the show is more or less set, you can use any table-sized material (cut up marley, yoga mat, sheet, sturdy plastic tarp) as your prop table map, and just lay it on whatever surface becomes the prop table in the venue of the day. Makes load-out easier, too - just roll or fold the cover, and stash it in the prop roadbox or case, instead of stripping multiple cards from a table.
If prop presets tend to change for each venue, I definitely see the value in cards or individual labels that can move - I always use spike tape labels, and just write them out on the back of my clipboard so I can stick them wherever the preset turns wants to be.