That new decal can add beauty and value. Its a matter of preference. Perhaps someone has already beat you to it. Maybe it looks sort of shabby and you decide it will look better off.  However you decide, if you peel it off you cant ever put it back on and look like new. I tried that with that orange sony sticker without much luck!

The beauty of a cassette player is to see the reels turning slowly, winding the tape.

That is missed with the decal on tape door.

But removing it (if ever, you are lucky to have a box with decal on it!) will degrade its value - as you said. 

Agh, don`t mention stickers. I have lost count, of the boxes i have had with stickers. Took them all off. I put them all in an envelope somewhere? I have left the sticker on my trk8800, only because, i can`t be bothered to take it off. I would like to find the sticker for the rx5500, as, i have never seen a pic with one on.