Is it safe to stack wireless triggers?

I just picked up a couple sets of RF-603's and wanted to make sure that I am able to stack them when I store them. I know that electronics are kinda funny when it comes to this stuff so I wanted to make sure that its safe. These have no batteries in them at the moment and I just did this to take pictures so you would know what I was trying to ask. Thanks!

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As long as they are off, shouldnt hurt them.

i do it all the time

Perfectly fine. I do the same as well.

Thanks for the replies. Just didnt want to harm them!