I voted sometimes.
If I can tell that the person I have written to has actually written on the envelope, whether it be my address or just adding the return address, I will keep it.
The envelopes which I sent, and only had my handwriting on, I'll throw away, although sometimes if they have the price on, I'll keep them for future records, so I know how much it cost them to send X amount of stuff back to me, so I can gauge how many dollars I should send to others to cover the postage.
It is amazing how many celebrities write their own envelopes; Ian McKellen, Michael Emerson, Jorge Garcia, Geoffrey Rush, Miranda Otto etc.
