A simplified version of the landing choices might be - if there's going to be sufficient propellant, the plan is for the F9 first stage to u-turn and go all the way back to the launch area to land.
If not, a drone ship is conveniently placed downrange so much less propellant is needed to land.
In either situation, are all the tanks topped-off to absolutely 100% of capacity "just in case", or are they filled only with the anticipated amount needed plus a safety margin?
The reason I'm asking what is actually done is that I seem to recall that "leftover" propellant might actually be helpful, since the thrust of the (three?) engines on landing is way more than the weight of an empty 1st stage, even when throttled.