I've always been a 'pinkie under' shooter...like 40 yrs of doing it that way. I've also made half a dozen grips for my Ruger Flat Tops to replace the cheese graters that came as standard. All have been slant grips except one pair that I am experimenting with.
Consistency of grip is a very big contributor to shooting groups. I've always found that mine drift vertically as a rule when I get sloppy with my grip. I concentrate on that aspect...a firm consistent grip, and that 'pinkie under' helps me position my hand in the same place, shot to shot. It does just that, but can hurt with heavy recoil or a grip with sharp edges, as the gun rotates in recoil.
With the full, flat bottomed grips, I find it harder to get the same placement shot to shot. Now that's me...and my hands... and 40 yrs of doing it that way. CaryC, the Pvt and most of the professional grip makers will tell you that the bulk of their business is Flat Bottomed. That says a lot...must be significant...as the guy who goes out and customizes his hand guns is generally not a 50 rds per year shooter. In a word, the experienced guys are buying Flat Bottomed grips! As I said before...my experimenting continues...
One last thought and its applicable to both types of grips. When using two hands, I've found that the weak hand, without conscious effort will exert add'l pressure to the grip...and that leads to unexplained group shifts. It's something I focus on, and it's a part of any pistol instruction I pass on to new or inexperienced shooters.
Below is a pic of my two handed grip...easy to increase pressure as the left hand fingers grip tighter or my knees squeeze tighter, especially with the Magnums. It's also my load development position. Sandbags don't work for me...and this one is a dead ringer for my deer stand position. A "cupped" weak hand position might minimize the add'l gripping tendency and its group shifting effect.
Just some thoughs, and hope I haven't hi-jacked this thread...no intent there...Rodfac