Hi John,
A full case certainly isn't "necessary" but it can help to avoid some position problems with some powders. If you're getting good ignition and uniform velocities, it isn't a problem regardless of the load density.
On the flip side, some slow burning powders will almost always result in compressed charges. This too, is not a problem, at least so far as shooting goes. At some point you may run into a situation where the load density is well over the case capacity if the charge is just thrown or run through a powder funnel. Good place to learn about drop tubes. This is a standard funnel, with the addition of a long tube. This allows the powder to better settle into the case as you load, sometimes drastically increasing the amount of powder you can get into what would otherwise be an overflowing case. Drop tube lengths of 4" to 6" are pretty common, and are available through any number of soucres (Sinclair, Forster, etc.), and are pretty inexpensive. For some cartridge/powder combinations, they're the only way to get a full charge in the case. Beyond that, so long as you're not damaging the bullet during the seating process, or the bullet isn't being pushed back out of the case by the powder, a compressed charge is just fine.
