Not as simple as that, but the answer is yes.

Not simple because "pressure" means different things, depending on what you are measuring, and why.   

If you mean peak pressure, it probably makes a difference.  What it can make a real difference in though, is the pressure curve, i.e., when the peak occurs, and how it ramps up to the peak and decays from the peak.   

The wrong position, i.e. held against the back of the case by a wad, generates a pressure wave that can generate a high pressure spike several times greater than the usual high pressure.   

Slumped in the case, but primarily at the back of the case gives the best and most consistent pressure curve.

Forward in the case gives erratic pressures.  This is probably due to poor/slow ignition, and powder and bullet being blown down the barrel before the pressure curve builds and allows the entire powder charge to ignite.

The best case scenario is if the powder is back against the case web, and the primer ignites the front of the powder charge.  Rocky Gibbs experimented with that extensively, and was able to get fantastic velocities without high pressures.  But, it took an intricate case with a tube from the primer to the front to ignite it, which for manufacturing reasons was impractical.   

In general, for our uses, tipping the rifle up to put the powder against the back of the case, and then going horizontal to firing position, allowing it to slump, is probably the best procedure overall.

Just a habit, give the cartridge rim a couple of taps on the bench to settle the powder then rotate it horizontal to load.
Don't know if it helps but makes me feel better.
Chuck

K, what’s a shot buffer? Kapok, a wad, cotton, cream cheese?

Thanks. Learning something new all the time. I looked it up and it did mention being used in center fire cartridges. So, powder, shot buffer over powder to fill up the case. Nothing between the powder and shot buffer?
That will save me cream cheese. 👍👍😂👌

Nothing between the buffer/filler if you use one and the powder.  If you put in a wad to separate them, it creates the flat front to the powder and a high pressure wave I talked about above, and can ring your barrel or cause it to burst.  Buffer/filler should be loose to allow the powder to intrude into it slightly, so it doesn't have that flat front.  That's why a rigid filler such as floral foam or such can also cause high pressure waves.

You want any buffer/filler you use to be very light, non-solid or ridgid, easily compressible and essentially "invisible" to the powder burning reaction.  So, putting a wad behind it is just like putting a wad in with no filler; it creates a high pressure wave in front of it, through the filler.

The goal with any method of holding the powder back toward the rear of the case is to not have a flat, confined front to the powder that can allow a resonant pressure wave to occur between the burning powder front and the bullet.

I use filler on one load.  I use ground up oat bran.  It's very light, adds maybe 1 grain to the load, and compresses to almost no space easily.  It's very similar to a commercial product that used to be available called puff-lon.