The issue of air space and fillers with BP have filled volumes of "expert" advice, without much in the way of definitive "best practices" ..... although there are a few hard rules on what not to do.

First, BP is most efficient and consistent when densely packed without contaminants. BP reached its zenith in small arms use with the British solid pellet used in the initial .303 loads pending the perfection of cordite. This loading developed pressures and bullet velocities in excess of twice what we think of when we talk BP cartridges.
Second, a little air space isn't a safety problem and in fact if one pulls down some factory ammo from back in the day one will find some air space in some cartridges. I've seen all sorts of percentages thrown around, but I've never seen anyone report serious issues with 20% or less airspace by volume.
The use of fillers and what type of filler to a large extent depends on the volume of the case involved and whether it is essentially straight walled (45-70) or necked (577/450). Natural fiber fill (cotton, kapok, carded wool) can be used in either type of case. Hard granulated fillers are purely for straight wall cases. Poly fillers are for those that enjoy scraping melted plastic out of their bores.

The main job of a filler is to hold the charge in place and ensure propagation of ignition from the base forward and prevent powder from being suspended in air before igniting (think coal dust explosion). This is crucial to some smokeless powders as well, where control of the surface area of ignition is critical to consistency if not safety. Here too is the point of the tighter the BP is packed, the more efficient the burn, giving greater energy and less fouling.

Adding granulated fillers in with the powder has been a practice as long as folks have looking for reduced BP loads. In a straight wall case there is no issue with pressures, but consistency will suffer since from a chemical perspective, the filler is a contaminate that interferes with the propagation of ignition. This will be evident in two ways, both readily shown with a chronograph. For example, 70 grains of pure BP with a fiber fill will give a higher velocity with a smaller spread in velocity than will 70 grains of BP mixed with a granular filler. Any contamination in a chemical burn, whether in your BP case or in the gas in your car will interfere with efficient ignition. That said, at the routine 100 yards the difference would not be readily discernible except to a match shooter. However since the burn is propelling much more solids than in straight BP, it is highly recommended to not use such in a necked cartridge.

The variations in consistency of ignition have to be major in order to be apparent to your casual 100-yard weekend plinker. As such to what extreme you take your reloading depends somewhat on what you plan to do with it. However since chronographs have become pretty inexpensive, anyone wishing to seriously investigate the properties of their loads should obtain one and use it regularly.

While as previously stated a little air space doesn't pose a problem regarding safety for normal use, one characteristic of BP ensures that all military loads were snuggly loaded, which is its propensity to grind itself to dust if loosely jostled around for a few years on horseback or similar long-term exposure to vibration.  As we all know, BP is graded on granulation and the finer it is the greater the pressure spike.  BP's weight, reliance on a simple mechanical bond and brittle nature mandated that no movement be allowed in the case least long-term transport result in a cartridge loaded with BP dust....a most explosive proposition.  You can readily prove this yourself by loading two cartridges (using spent primers).  One with a 50% load of BP and the other with a 50% load plus filler.  Place in your vibrating case cleaner overnight and then unload the cartridges in the morning and compare granulation and dust content.  The loose load will be ground down with significant dust.  The powder from the load with filler will be largely unchanged.  While this doesn't come into play for our normal use, the military had to assume worst case scenarios....particularly when ammo was being carried half a world away and then hauled around for who knows how long before use.