Going to be mounting new slicks in the next week or two. Tubeless, of course, and I was wondering if the practice of soaping the inside really makes a difference in how well they hold air. I know it will matter to some, the tires are Phoenix brand 14 x 32 - 15. Thank you for any input.

I soap my tires when I mount them. Do I have some kind of scientific evidence that it makes a difference - no.
My perception is that it helps the bead seat and seal, and I mount all my own stuff for the classic and race cars I have now.

I also use a soap solution when mounting tires. Specifically, I was referring to the practice of coating the inside with dish soap to reduce air loss thru the sidewalls.

I use baby oil on the inside of the tire to keep the sidewalls from leaking air if no tubes are going to be used. If tubes are to be used, then corn starch to keep from pinching the tubes when the tires wrinkle on launch.

Dawn dish soap helps for sure. Glop it on inner side walls while laying flat. Let it dry a bit. Then flip the tires over and do the other sides. M/T 's will leak badly w/o soap. I've had them only lose 5-6 lbs setting over the winter with this method.
Doug

Dawn liquid here as well Definitely part of the new slick routine RC

Baby powder works well also to seal the tubeless slicks as well as with tubes, I use a soapy solution on the beads when mounting them here at home on my old manual tire machine, I inflate them to the same circumstance + or -1/4 inch circumference on the O.D. measure in the middle of the tires, inflate them above 15.0 Lb. up to 25.0 Lb. to make them as large as a I can on the O.D. for 3 to 5 days before racing them at the lower race pressures

I used to really soap mine down, now I don't. The results for me is exactly the same. Radials or bias. I kept a big paint brush and bought Palmolive by the 64 oz jug to do this.

Tubeless get soaped here...doesn't stop them from airing down but greatly lengthens the time - From days to months.

Murphy's oil soap works great on tubeless slicks.

We Dawn dish soap ours a night or two before we take them to be mounted and then let the person mounting them soap them up again. They hold air really well that way.

If you have to soap a tire so it holds air, the tire is defective. If you buy a brand that needs soaped every time, you're buying a defective brand.