Usually the receiver rails are more of an issue with slide frame fit. People use accurail or have the receiver rails micro-welded and re-machined to fit the slide.
However, the contact between the barrel bushing/barrel, Upper Lug/Barrel Hood, and lower lug to slide stop pin are most important to accuracy. If these are good, the slide will tighten up pretty well in battery. Jerry Kuhnhausens book places a 15% accuracy improvement on slide tightness.
I shoot Bullseye/Precision Pistol and have a Les Baer with a fairly loose slide (From 70,000+ rounds) I will still hold the 10 ring at 50 yards
Keith