In all cases always ask the CC if you could use it, if you want to avoid unnecessary problems!
Why, I wonder. They are electronically inert, as someone has pointed out. In other words they represent no more risk to aircraft safety than a Biro pen, and perhaps less than an Ipod.
If the reason is that the CC are not aware of that and must not be alarmed, it would be a good idea to tell them.
It's far too easy to say "Ask permission", when doing so is actually totally unnecessary. You'll be recommending asking permission to read a book next, just in case the CC think that's a dangerous activity.
Of course GPS usually doesn't work in the cabin, and the MAPSHIFT problem is a real one. But that's not the point! There's no reason to stop anyone trying.