.38's in a .357...
Silvanus--First of all, welcome to The Magnificent Obsession--Reloading!
And, BTW, the only stupid question is the one not asked.
Now, as to yr question, I do exactly that with my target loads. I shoot bullseye with my .357 S&W revolver, and rather than mess with having .38 cases around in addition to .357 cases, I load everything in .357 cases.
Theoretically, you'd need a small amount more powder in the .357 case to achieve the exact same bullet velocity as you'd get with a .38 case (all else being equal).
However with my bullseye wadcutter loads, for practice and competition, I just use the same load, a rather light one, and my S&W likes 'em just fine. The ammo is more accurate than I am.
For hunting, I use the same .357 cases, a much heavier bullet, and a slower-burning powder. The gun barks louder, recoils more, and of course hits deer harder. (And, FWIW, the cases last for a lot fewer reloading cycles!)
Bottom line: It depends on what you're loading for. For target and practice, which should be the vast majority of any handgunner's shooting, you load light target loads to the level yr handgun likes. Keeping within the parameters of a loading manual of course. Regardless of which cases you use.
So, while loading a .357 to .38 levels works just fine, of course you would NOT load a .38 case to .357 levels--you'd probably be endangering the .38 pistol in question, not to mention yr own fingers and eyes.