The announcement of the new scented stamps to be released soon from the U.S.P.S. has prompted a lot of discussion involving allergies and chemical sensitivities. (Wow, the Facebook comments. Yeesh. And people ask why we don't have a Facebook account.) Here at L.W.A.H.Q. we promote an "Ask First" policy in regards to scented mail. Especially in those new pen pal relationships, you should avoid using perfumed paper or ink before you know whether or not your pal has sensitivities to such things. We have honestly had members contact us because they couldn't open a letter due to the smell. It's not something you might think about; so we're asking for awareness and courtesy. If you're going to want to use the scented stamps or add fragrance to your mail, let your pals know so they can opt-out if they need to.
P.S. And I personally think you should ask before using glitter as well. That stuff's dangerous. Someone could lose an eye. (Fun fact: my grandma had to go to the hospital following a glitter incident. I've been a bit touchy about it since, and make a point to use non-shedding glitter tape in my mail art when I use it at all.)