The pinion angle must always be checked....no matter what you are working on.
And no one said to install the pinion pointing backwards...I DID say to install the springs backwards, which I have seen done, and it results in the pinion pointing down to the ground too much because the shims are facing the wrong way, and on most vehicles, it will also move the axle backwards, cuz the front part of the spring is shorter than the rear part.
Looking at the pic you posted a link to....well, I see 1 aluminum block, with a line in it on the side, but not on the front, so it is a 1 piece block There is the stock cast iron overlo on top of that, which has the extension piece on it to contact the bump stop when the spring is overloaded, and above that, attached to the spring, is the shim which corrects the pinion angle for the amount of lift.
Why do you think those shims are put on the bottom of the springs by the spring manufacturer or lift kit manufacturer ??? It is because they have to be there to get the pinion angle right.....so maybe it IS something to worry about, cuz I am sure they didn't put it in there just to waste metal and money.
And in the pic you posted with the modified 3 link...well, for the type of driveshaft being used, your pinion angles are fine. But, it is not the slipjoint that makes it that way, it is the double cardan joint. You have 0 deg at the hogshead, and 15 deg at the tcase (15 deg because you have a double cardan joint which splits the 30 degs in half). That works for a slipjointed double cardan shaft, but not for a regular slipjointed shaft. A double cardan joint driveshaft needs 0 deg at one end, at least that is what the company who made my custom slip jointed double cardan driveshafts told me (Denny's Driveshafts).
And in that setup, the pinion angle is nowhere close to stock, nor is the suspension for that matter, so apparently someone worried about it enough to rotate the pinion up to get it the way it is when they built the suspension.
The slipjoint driveshaft is more tolerant of non-matching pinion angles, but they still must be checked to avoid problems. You can not run 0 deg at one end and 15 deg at the other end without wear issues unless you have a slipjoint AND a double cardan joint.
A regular slipjoint shaft does not have the double cardan joint in it, and requires the matching pinion angles to avoid wearing out the ujoints. The older dodge trucks used slip joint rear driveshafts, but had no double cardan joint, and as such, you had to check pinion angle when lifting them and use shims to get the pinion right.
I had a problem with my front driveshaft on my 79 dodge ramcharger which had a slipjoint driveshaft and a double cardan joint because the alignment shop took the shims out from uinder the springs when doing the alignment. SO, if you don't need to worry about the pinion angle with a slipjoint shaft as you claim, then why did I have binding with the shims out ???