We recommend it.  The quality of our paper makes most of our invitations too thick to make it through the post office stamping machine cleanly.  They get wrinkled, sometimes it takes off some of the graphics,  and the ink of the stamp smears and makes the envelope messy.   Keep in mind that some offices are understaffed and may not have time to hand cancel a large number of invitations, however, they may allow you to hand cancel them yourself.  If you do not get help from one office you may consider trying another, preferably in a smaller location as they tend to be a little more friendly.