It will prevent the stamp from being handled by the automatic sorting machines. That will slow it down getting to its destination as it must be sorted by hand.
Many envelopes will have a box in the top right corner which says something about the postage. If does not have that, you usually place the stamp in the top right corner of the envelope. If you have issues, then you should go to your Post Office.
A stamp.
a stamp
A STAMP
a stamp
A postage stamp.
postage stamp
A stamp goes around the world but stays in a corner.
A stamp on a post card.
A postage stamp
A Stamp
A STAMP