I'm 100% open about it. Especially buying merchandise, as over the last few years since XY, really the merch has been everywhere, they don't expect to sell that while making fun of you at the same time. :P
I always carry a Nidoking key chain, which I recently switched out for the tomy Piplup plush key chain Hot topic sells. Most people don't know what it even is, but when they do you know they actually know Pokemon somewhat.
My only thing, is that I won't wear/carry Pikachu things or things with the English logo, especially rather large. The things non-fans know in an instant, and usually these are the ones you don't need comments from. I'm not a fan of either, and it's just asking for that older cashier to say something rude. :/ Yeah, pretty sure she doesn't know what the heck my Arcanine shirt is at all.
Pikachu shirt with logos and it's all "Aren't you too old for that?" right off the bat.
I've had many an instance like that with an older cashier anywhere, from Dairy Queen, to the bank with 'things to say' about that, so that comes from a lot of actual experience. What series I like really shouldn't be something to criticize your customer over, but that's just how it is. No reason to over advertise it either.