firm shifts are better for the trans. like bush said, slippage occurs during a shift. This generates heat and wears clutches faster, which leads to more frequent fluid changes...
If there's even a remote chance of me having to accelerate quickly (on ramps, passing, etc) I use tow/haul. it also holds the gear longer before shifting, which usually means a faster sync between the two gears and a quicker shift...
Heat and slippage are the two things that kill autos the quickest.
PSD