Lavan,
As you might gather, the answer to your question is, "it depends". There are both gun and bullet reasons for leading. One gun cause that has gone unmentioned can actually result in softer bullets leading less than hard ones. That is the unfortunately common situation where the barrel and frame threads of a revolver are tight enough to constrict the barrel at the throat by up to several thousandths of an inch. This situation causes the bullet to be narrowed passing through the constriction. In this instance, a hard cast bullet frequently does not have enough spring nor is under enough pressure to upset enough to maintain bore obturation after passing through the constriction. So, once past the constriction, gas cutting commences, blast spraying lead onto the bore surface and unbalancing the bullet to produce large groups. In this one instance, if the bore is smooth, a soft lead bullet can often bump back up under acceleration pressure to maintain obturation of the bore while squeezing through and again after the constriction.
If you develop leading, do not succumb to the tempation to shoot it out with a jacketed bullet. Manufacturers warn that this raises pressure because the soft lead lubricates the jacketed bullet so it tends slide over top of the lead rather than push it out of the way, and therefore to swell the lead-tightened bore while ironing or burnishing the lead into its surface. Lots of folks who do this (I was one back in the '80s before I figured out what was happening) will tell you the bore looks clean and smooth afterward "shooting the lead out". It does. The lead glazing is quite shiny and reflective with light hitting it at a highly oblique angle, as it does looking through a bore. But the lead is there. I discovered this in one of my revolvers when I had the bore smooth from jacketed bullet firing and patching. But the patches kept coming out a little gray, so I ran a brush through. Looking again, the formerly smooth and polished bore suddenly had all these large dull areas. They were lead whose surface the brush had scratched. Back then we used mercury to get lead out, and a good bit of the resulting crusty amalgam brushed out after a soak in it. Today, much safer, is the No-Lead product from the Wipe-Out makers. It sits in the gun for an hour and makes a new compound with the lead which patches out quite easily. No brushing needed.
Firelapping will remove constrictions and smooth a bore, and if you have a rough bore that slugs uniform and without tight spots, you can skip the coarsest abrasive grade in the kit and shoot just the polishing grades through it. Indeed, Merrill Martin had a series of photos in Precision Shooting years ago wherein he sandbagged a revolver through 18 firelapping rounds, firing six each onto separate targets. The first target was pretty wide. The second didn't look much different. The third group was way tighter. So, regardless of the condition your bore starts in, there are ways to address the problem.