Whenever you fish a lure that requires a straight fall down, when do you tend to close the bail/ activate reel? On a slow falling weightless rig do you keep the bail open until it hits bottom, and if so can you detect bites on the fall?

On a spinning reel, when it hits the water, I'll do a tip sweep from level to overhead with the bail open to let out enough line for the lure to fall straight down...then close the bail.

 

Baitcaster, I'll manually haul out a length of line before engaging the reel.

I engage the spool once the bait hits bottom or when a fish bites, whichever comes first. You can still detect bites while it is falling.

 

I do a lot of tip sweeping that @MN Fisher describes too. 

Being a Senko fisherman, you learn to line watch.  I watch 2 areas of my line.  First where the bait entered the water and where the line come out of the water closest to the tip of my rod.  I either lift the rod with the bail open to have enough line out where it falls without resistance or I leave the bail open.  Depends on depth I am getting down to. 

I’ll cast and once the lure hits the water I’ll raise my rod above the plane of my head then engage the spool letting the lure drop straight down. This is for pitching. If I’m fishing a bit deeper I’ll do the same and pull line out then start my retrieve. I’m always watching my line for movement.