Saying 'rest' in peace, or more often writing R.I.P after a deceased person's name, is primarily a way of showing respect to the dead person and the feelings of their relatives.
Like so much of What we do ( toasts etc) Its nothing to do with belief and everything to do with the feelings of others ie toasting the Bride and groom is just showing a shared happiness and showing you wish them well on their wedding day.
Watchtower seems to use all these micro manage behaviours to take every opportunity to make ordinary jw's feel different and put a big, rude divide between them and non jw's. Its cruel.