I have done 15 - 20 boxes of quarters and found 3 silvers + 1 silver canadian. I have found 4 silver quarters sitting in a teller tray at the bottom all shiny, im like what are those? And got em for $1. As many have said before, quarters are easy to notice with that shiny silver edge in a teller tray. I am sure all tellers know to grab those, and they hardly make it back to rolls/boxes. Also, John Q Public will spot that old quarter & snipe it in pocket changes easily.
I have had much better luck in bags of quarters from machines that typically do not reject silver, finding also 4 or 5 in my best bag. By the time those bags get mixed together with everything else at the coin carrier, we get that 1 silver per 5-10 box average.
That being said quarters are easiest to obtain & dump.
As far as dimes go, I have had tellers tell me "Are you looking for silver? You aren't going to find silver here" asking for halves, and then I'm having her fork over all those shiny dimes (silvers). People don't look/know about the dimes.