I find that I use phrases, coupled with "Play On" like:
"No contest" - when the throw is perhaps slightly too askew, but good enough to be let go when there is no contest. I'll often follow this with a word at the next lineout to the offending hooker asking for it straighter, as he would have been called if they contested it, or if it was any more askew.
When there's been "accidental offside" or "crossing" but no defender - I'll often call "No tackler" or something simliar.
The idea being - I'm communicating that I have seen the "offence", but deem it immaterial due to the actions or inactions fo the "non-offending" team. Never had any objections to it from players, assessors, spectators - and even had a few players say they liked it.
While I agree players should play to the whistle, find that calling "play on" signifies to the players that there ISN'T going to be a whistle.