Lots of good experiences and info here. I'll add my $.02:

First - Primers:

There is a big difference in primers between rifle and handgun regarding when to use magnums and standards. In rifles, there is a difference in cup thickness between standards and magnums (as well as the force of the primer), so when you're loading a high-pressure cartridge (mostly, but not always, also called "magnum") you want a magnum primer. Pistol primers are different - no difference in cup thickness between standard and magnum; but there is a big difference in the flame temp, flame duration/length, and brisance. SO, in pistols, the primer difference is more about the conditions of the load, whereas in rifles it's partially about the conditions of the load, but also about the primer handling pressures properly for the load being used. This confuses people, generally when they have a lot of experience with one type of gun and then start loading the other type and use their past experience to apply to the new situation.

WLPs are indeed marked "For standard AND magnum". They have been tested and they are in-between a standard and magnum primer for the most part (might lean a little towards the magnum end).

Magnum primers are called for in pistols for three circumstances: Cold weather, ball powders, and heavy loads of powder. These are guidelines and not hard-and-fast rules. For example, some ball powders have heavier deterrent coatings than others, so some ball powders work fine with standard primers.

Generally speaking, most cartridges with most powders work fine with standard primers in most loadings. I use WLPs exclusively for all my LP primer loading, because it saves on inventory, and Winchester factory ammo doesn't seem to suffer from using the "in-between" primer in situations that don't call for such, for example the 45 ACP. (Never a need for a magnum LP primer there, but the WLP doesn't have a detrimental effect.) This has given me the impression that Winchester has discovered a "sweet spot" when it comes to handgun LP primers.

As far as the OP's question about 357 loads with W231, I have more than a little experience. I think for the most part it's been covered pretty well by the posters above. I, too, get better results using the old Winchester-published data (rather than the Hodgdon-published data we're stuck with today...).

I do NOT use W231 for "real Magnum" loads, rather, I use it to achieve 38-44 equivalent loads using cast bullets. I don't care for the 125gr bullets in 357, but if I were to load them, and were to use W231, I like the 7.5gr load. I use 140gr to 160gr bullets (jacketed, cast, and swaged) in the 357 with both W231 and AA#9 or 2400. I load the 38-equivalent and 38-44-type loads with the W231 and the "real Magnum" loads with the slower-burners, but W231 is a totally acceptable and useful powder for the 357.