I'm brand new to this forum, but an old hand at BP shooting. I started with cap 'n ball revolvers in 1970 and like them better than modern fire arms.
Several observations regarding the wad/grease/tight ball/ chain fire issue. First, some sort of grease over the cylinder is the current best and most accepted prevent measure regarding chain fires. That being said, there are other considerations.
First, history. Back in the Civil War these revolvers were rarely loaded with loose powder and ball. They used combustable paper cartridges. That's what I use today, and I roll 'em myself. I cast my own bullets using the Lee .450 double groove mold. I treat cigarette paper with potassium nitrate and roll them into tapered cartridges, closing the bottom of the cartridge over the end of the rolling dowel with a fold and a dab of Elmers white glue, load in .30 of fffg, melt bees wax into the grooves, and use the residual wax on the bullet to attach the cartridge to the base ring. I trim off the excess with a razor blade. This is exactly how the originals were made more or less. There is (was) no provision for a wad between the slug and the charge. So, what did they do in the field? Depended on the situation. If the shooter got himself into a reload situation he probably dispensed with the grease step and depended upon the tightness of the bullet and the wax in the grooves to protect against chain fires. If it was General Lee riding about on Traveler, rarely if ever actually shooting the weapon, the weapon was handed to an orderly who reloaded it, and probably added some axle grease or tallow to seal the chambers. Truth is, most were not sealed with anything other than the wax in the grooves.
But, like everyone else these days, I do use loose powder once in a while, I load with both a loader or the guns own ram, I carry extra cylinders in my belt, I use round ball if it's all I have, and I even use conversion cylinders with .44-40 cartridges, sometimes both, sometimes all of this stuff! I use Crisco, 3 in 1 oil, grease, patch lube, depending on what I have. However, I have never put a wad between the load and the bullet, but when I use the combustable cartridges I have used a Cabellas pistol wad on top of the bullet. I get pretty good groups, dependability, and cleaner firing.
I did get a chain fire once, back in '77. It came from a cap falling off a nipple. I had a bad batch of caps, many were falling off, and I got careless and tried to use 'em anyhow. This chain fire came in spite of the cylinders being crisco'd over, during a NSSA match, and caused a great deal of concern, finger pointing, shouting, accusiations, an investigation a few weeks later, and ultimately my leaving the event early!
That was before I started using paper cartridges.
It takes time to roll and assemble cartridges, but I have lots of time these days. Hell, I used to roll 'em at work while waiting for customers to come in!
Short answer, wads on top won't hurt, but there ain't any prevent for what carelessness might cause.