Even if that is someone's intention, I believe it a mistake to say that phrase publicly. So, for instance, if when Rush Limbaugh died, we would post RIP, the impression is one believes that there's a real possibility (even likelihood) that the person is saved ... which would be scandalous. Nobody reads into the phrase the intent that you describe above, so it could be an occasion for scandal to say it without explicitly adding the qualification you stipulate. And, if you're unwilling to explicitly state your mind as above, then it's likely that you're intentionally leaving the meaning ambiguous, so it could be taken in an orthodox sense by Catholics and interpreted by others in their religious indifferentist sense by non-Catholics.
I don't think it's ever a bad thing to say these words. When one says "may he rest in peace", the very construction of that phrase indicates that our prayer is needed --- and it is indeed a prayer, I'm saying it to Almighty God, not to my neighbor --- and that there is the possibility that they are
not "resting in peace". If they're in heaven, they don't need that prayer. If their salvation was in jeopardy, or if they are serving a long sojourn in purgatory, then that prayer most certainly
is needed. (It's just a coincidence that the abbreviation RIP stands for the same thing in Latin and English, though nobody outside of Catholic and Latinist circles comprehends this. And, granted, "rest in peace" is not the same thing as "
may he rest in peace".)
The phrase that I
really dislike is "they're in a better place now". I just keep silence when someone says that --- I never, ever say it myself, in that I am not a Pope and I cannot canonize anyone. I want to say "let's hope so", but then that would introduce an element of doubt, and in that moderns are all basically Unitarian Universalists when it comes to questions of salvation and immediate entry into heaven, it could provoke verbally violent reactions. I normally just tell people, usually non-Catholics (I actually do not talk to that many Catholics, I live in an overwhelming Protestant part of the country, and to tell you the truth, I really don't
want to talk to post-Vatican II "Newchurch" Catholics in the first place!), that I'm an "old-fashioned", "old-school", "traditional" Catholic, then I give some kind of litany about "you know, rosary, scapular, Latin Mass, fish on Friday, and so on". If they want to know further what I believe, they can look it up. I do comment, as appropriate, that "we're quite a bit more conservative than the parish church (or school) down the road". If there is a way to get and keep their attention and interest beyond that, I'd like to know about it. At some point I just have to let go and let the Holy Ghost take over.