all revolver rounds leave a ring of "goo" at the end of the case when fired.
Since 38s are a tad shorter then357s, the 357 goes past that ring left by the 38.
When the revolver is fired the case expands. That ring of goo left by the 38 crabs the longer case, making it hard to extract.
Cleaning out the goo ring left by the 38 needs to be done before shooting the 357.
I personally don't worry about it, I shoot 38s out of my 38 Revolvers and 357 out of my 357 revolvers, but most people shoot both out of the same gun (that's the way they were designed).
There are several ways to do this. Lots of scrubbing with a brush, using a Lewis Lead Remover, etc.
All in my opinion are a pain in the behind.
I take a 357 case, bell it with the expander die to where it has to be forced in the cylinder. Then I force the empty case in the chamber which cuts out the goo ring left by the 38. Followed by normal cleaning.
Before shooting 38s, put oil in each cylinder, it makes the "goo" ring come out easier no mater what method you use to clean the cylinder.
If I were to shoot a lot of 38s, I'd oil the cylinders ever few rounds.
If you're using your 357 in events such as Bowling pins or something that may require a fast reload, I wouldn't even shoot 38s in it.
I'm a huge fan of easy extraction of my revolvers.
A long the same lines, cylinders get dirty even if you shoot 38s in a 38 or only 357s in 357. I like to keep my cylinders clean to the point where if the cylinder is open and turned upside down, the fired brass comes out under their own weight and gravity.
This is critical if you shoot ICORE or similar games. Handy in SD if you might need to do a fast reload also.
I carried a Model 28 in my 20 year LE career. I was a fanatic about keeping my cylinders clean. I wouldn't shoot 38s in my service revolver. If for some reason I wanted light 38 loads in this gun, I'd load light loads in my 357 cases. But I ALWAYS used different bullets then I used in my service ammo.
Wouldn't pay to get them mixed up.