Well, if you want to give them a rest, better remove them from the slots. Put them in a nice, dry box and check upon them regularly, making sure they won't corrode. Once you think they're well-rested, you can insert them back in those slots again and they will work just as hard as before. :-)
Oh, just kidding. Since you added more RAM, it's likely that they will be used less often since the memory is now divided over four DIMMS. But the OS will claim more RAM, simply because you have more. Since the processor will just map the RAM to whatever location it likes, there's just no clear way to tell which slot will be preferred more than others.