In general, you only have to peen rails if you are using an optical sight that engages to the frame, because then sight and barrel registration then depend on it.
Starting with a GI barrel with weld build-up of the barrel extension and link lugs and a loose slide and frame, I was taught that tightening the slide and frame only accounted for about 5% of the overall accuracy improvement. That's a small enough number that you have to test for it statistically, so the number is suspect, as I doubt the military match team armorers I got it from were applying a stringent test of any kind to it. They may have been able to tell from cumulative scores, based on increases in the number of scratch points and X's over several seasons of matches. But the human variables, including psychological positives from having a tighter feeling gun, are so much larger, I'd have a hard time being confident in the analysis of that kind of data.
Whatever the difference may be, it's probably less with the max OD barrels you buy for fitting up a gun now. There's also been a long running argument about whether tighter slide and frame fit improves function reliability or lessens it. It's a little humorous if you try to follow it. The pro side argues the more consistent motion is better. The con side says the tight fit is prone to be vulnerable to dirt and grit. The pro side asks if the con side is planning to revive trench warfare, and if not, why would the old wet sand function test be relevant? The con side say no, fouling can cause grief too, etcetera, etcetera.
Since I was taught to peen rails thirty years ago, I generally did it and never had trouble with it on forged receivers. But I have refused to try it on cast and stainless receivers, for fear of causing the stress cracking Bob referred to and for concern about galling with tight fitting stainless (though the alloys seem to have gotten better about this over time, as compared to early stainless guns). I've had a couple of 25 yard one-holers (all holes overlapping somewhere on the paper) that were built the old way (weld-up and filing and scraping), and shooting into about 3/8 inch. Both were on peened Colt series '70 frames. Both shot those groups off an old steel Hoppe's rest, and not a machine rest. In retrospect, my scientific side wishes I had left them unpeened, shot them for group, then peened them and refit the barrel to the lowered slide to try to get hard numbers on any difference it made. But I also had a rock solid hold and better than 20/10 vision back then, and now I don't. So I doubt that I could any longer tell reliably by that method.
I can say that I've put over 3,000 rounds of sooty, lubricated lead bullets fired by Bullseye through a peened frame gun without cleaning before it started to fail to go all the way into battery on every loading cycle. So I don't think the fouling argument has weight when the powder doesn't produce gritty residue. I'm not confident that some of the slower spherical propellants would let me do that, though.