minimum-length and set-spacing-rods

[-Eluze]

NR   5.4.6 Spanners     Using the
spanner-interface   wrote:
> 
> For some layout objects, the minimum-length property becomes effective
> only if the set-spacing-rods procedure is called explicitly. To do this,
> the springs-and-rods property should be set to
> ly:spanner::set-spacing-rods. 
> 
… and then a few examples follow to demonstrate when it works by itself

- tie
- MultiMeasureRest
- Hairpin

and a few examples (2) showing when you have to use \once \override …
#'springs-and-rods = #ly:spanner::set-spacing-rods to make it effective:

- Glissando
- Beam

this raises a few questions:

- is there a list of grobs requiring to call set-spacing-rods explicitely?
- is it important to use \once (and which side-effects happen if you do
not)?
- could it be implemented/automated?

thanks
Eluze
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