Dan, I try to avoid fillers, but I am not dead set against them. My usual practice is to start off with loads of the faster rifle powders that will give me roughly 1800 fps. If I can find a load that gives me better than inch at 50 yard groups from scoped rifles, I stop there.
It's when the groups hover around 1.25 to 1.50 that I break out the dacron fiber. I never use more than a grain and a half and I use it fluffed up to fill the air space over a charge one grain less than my best load without it to start. That can often settle down a load that is almost good enough.
My Chrony shows that dacron does raise the pressures a little. With cast bullet loads this is not a safety concern, but it is an accuracy concern. I have come to believe that CB accuracy is closely related to matching pressure to the alloy used. A minor change in pressure can cause a major change in accuracy.