Quoted:
As others have said:
Top round on left; easier on thumb for left handers.
Top round on right; easier on thumb for right handers.
The rifle don't care!!! I don't pay particular attention which way I load my clips. Works both ways!!
'Mo

From “The M1 Rifle” page 14 published by the NRA

M1 7th Round Jam
While loading some 8-round M1 clips for a club match, I was warned to load them with the top cartridge on the right, or risk failures of the rifles to function. Is there any substance to this?

Answer: The warning you received was exactly backwards. The problem described did exist, briefly, but was solved sometime in late 1940 or early 1941. Further, before the fix was introduced, loading an M1 clip with the top round on the right did not prevent the rifle from jamming, it caused a jam to occur. The problem was that when a rifle was loaded with a clip that had the top round on the right, the seventh round (also on the right) would jump the clip and stub against the rear of the barrel above the chamber. The condition was considered so serious (and potentially damaging to the already questioned reputation of the Army's new rifle) that the M1 Rifles sent to Camp Perry in 1940 were specially modified to prevent them from being loaded with a clip having the top round on the right. In addition, ammunition sent with the rifles was checked and any clip found to have the top round on the right was reloaded to put it on the left. The problem was solved shortly after the 1940 National Matches when engineers at the Springfield Armory found that a drilling operation removed metal from two vertical ribs that supported the left front edge of the clip. Restoring the two ribs to their original configuration stopped the 7th round jam from occurring and allowed ammunition manufacturers to load clips without regard the location of the top cartridge. As a matter of convenience, it is easier for a right-handed shooter to load a clip that has the top round on the right.

From the M1 Rifle field manual FM 23-5 http://www.fulton-armory.com/tea/op.htm

quote:


14. Loading Rounds into a Clip

(a.) Insert eight rounds into the clip, holding the clip, open end upward, and the rounds in palm of your left hand. Start placing the rounds in from the lower left of the clip and make sure that each round is against the rear wall so that the inner rib of the clip engages the extracting groove of each round. The top round will then be on the right, making the clip easier for a right handed firer to load in the rifle. For the same reason, clips are loaded this way at arsenals.

(b.) Each time rounds are loaded into a clip, the clip should be checked for long rounds. If one rounds extends beyond the others, it will be hard to load the clip into the rifle. The long round should be seated by removing the top round, pushing the long round into place and then replacing the top round. Tapping the bullet against a solid surface to seat the long round may result in the bullet being pushed back into its cartridge case. This may damage the bullet or break the bullet seal which could result in changes in the ballistic performance of the round.

Quoted:
I don't pay any attention to it when I load them and have had no feed problems.

Quoted:


Go shoot a garand clinic.  They tend to show a good way to load the clip.  For a righty, put the meat of the palm of your hand on the "hook" for the charging handle.  Then you can drive the clip home with your thumb and if the bolt starts moving forward, your meat of your palm will prevent the bolt from closing on your finger.  While inserting the clip, also try to arch your thumb backwards.  With a little practice, you can load and remove your thumb and minimize any chance of getting garand thumb.