I would suggest you apply CA wherever you suspect the integrity of the 
plane has been compromised by wedge/iron and hammering - splits 
resulting from normal use or abuse. That includes the splitting you 
describe on the smoother. Those splits can sometimes be helped with 
screws or, more radically, by planing the side(s) off to remove the 
splits, attaching new wood, and shaping as required.  Only do one side 
at a time! ;-)

Any splits resulting simply from wood movement should be considered 
individually. Are they resulting in defomation of the plane body? Can 
they be readily closed by clamping without causing deformation? Are they 
new cracks or old? Have you observed their elongation since you've owned 
the planes? If left alone, are they likely to extend to the mouth or 
affect the wedging action?

I would only apply BLO after finishing with any gluing, and likely then 
only to close up end grain checking visible in the mouth or ends of the 
body. It's mostly cosmetic IMO.

FWIW
Don

On 2015-04-23 1:01 PM, Mark Jefferis wrote:
> On the jack plane, I am concerned that the number, placement, length 
> and width of the checks/cracks imply that the checks/cracks could 
> lengthen and break the body a part. I am imagining that stabilizing 
> these cracks could prevent them from growing. I am not worried about 
> appearance. I just don't want it to fall a part while I'm using it.
>
> Of most concern, the smoother has length wise cracks/splits on each 
> side of the body top where the wedge terminates in the throat against 
> the iron. These cracks in front of the wedge are directly affected by 
> the wedge's pressure. The cracks follow the grain. I am able to close 
> these two splits with a pliers.
> Is there agreement that CA glue is an effective and easy approach to 
> preventing the two wooden planes from falling a part? Initially I was 
> going to use yellow wood glue.
>
> I will also take up the suggestion about soaking them in BLO.