I like to thing of this way
0%-80 or 90%- Fsmash can work for a good damage build up. I commonly will use this quite a bit early in the match (not spam so don't think it) in conjunction with banana trips, grabs, and utilt uair hits.
80 or 90% - 120 or 130% -This is when you need to prepare for the kill, fsmash is stale but dsmash doesn't have as much range so you must be prepared with both. That being said, when it gets to these high percentages, my dtilt and ftilt start becoming more prevailent along with my usmash and my utilt. That along with gratuitous amounts of pummeling. All of this is done with the sole purpose of refreshing kill moves. (Bananas are always a constant of course.)
130%- KO - This is when the fsmash re-emerges with it's awesome counterpart, the dsmash. The bananas are now, as always your kill setups, so long as you can spring that dsmash at the right time or whatnot, you will do fine. If their damage gets obnoxiously high, then theres probably a chance that your smashs are no longer fresh and that somehow they survived, if thats the case, you most likely can kill with uair, ftilt, or utilt.
This is the general way I think about it, I don't see a point in 'saving your kill move' if you aren't in kill range. That being said, I think it's entirely possible to use fsmash all the way up until they are in kill range, then a few hits later from other attacks, and your fsmash is your kill move once again. Refreshing an attack can happen in a matter of seconds, and diddy is amazing at setting up smash kills with his bananas. That being said, there shouldn't be many worries about stale moves so long as when the time comes, you prepare yourself.
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