The whole issue of unregulated wall warts is a point of confusion for many beginners. It's not about distrusting the lable but understanding what the lable means. An unregulated DC WW rated for, say, 12V at 1A will read quite a bit above 12V when unloaded. It should deliver at least 12V when loaded at 1A, though. That's why it's not such a simple selection. When feeding a regulator, either choose a higher voltage or higher current rating. Even then, measure the voltage under load to be sure that you are feeding your regulator with a high enough voltage.