From what I understand it was a mix of the two lengths.
Primarily whenever the whip was coiled and attached to his belt it was the 10 foot. The reason being that it typically looks fuller when coiled because of the extra length.
For the shot where he removes it from his belt in the streets of Cairo the close up shows Harrison pulling the 10 foot from his hip. This is the same in the shot at the beginning of the film in the jungle. When they cut to the wide shots Harrison is actually cracking the 8 ft. One of the tells is that depending on the camera angle you can see that they filmed those shots with a long lens and the street isn't as wide as it appears in other shots. Likewise in the jungle scene from the beginning they are filming under a thick canopy and a shorter whip length would have been necessary to prevent it from getting caught in tree limbs. When they cut back to Indy angrily coiling the whip back up, it is again changed back to the 10 foot.
The shot where you are looking from Marion's perspective from inside the horse drawn carriage in Cairo (and you see Indy from the back cracking the whip) it's clear that the street is fairly narrow which would make it difficult for him to crack a 10 foot in such a narrow space. By using a long lens it gives the illusion that the street is bigger than it actually is. Notice how the wide the street looks when you see Indy from the front when he does his "Cairo Flash" and by contrast how narrow it looks when you see Indy from the back (from Marion's point of view.)
One scene that he is definitely cracking the 10 foot is the deleted scene between him and the swordsman. They are filming in a much wider street and Harrison has more room to move. He is even running up a few paces to antagonize the swordsman. The space is very open compared to the narrower streets in other shots.
As a general rule whenever the whip is in close up/ or being removed from the belt it is the 10 foot. Whenever the whip is being cracked/ or doing wraps/ more complex moves/ the 8 foot is used. For the wrapping and swinging scenes the 10-12 foot is used. Though when Harrison or the stunt team is actually swinging, they aren't even using whips at all. Those were special stunt wires that had braided leather over them to appear as whips, often with the wire extending out of the end of the braid several feet to be secured to whatever object they are swinging from.
I think the confusion comes in when it comes to different camera angles and lens lengths. The scene in Last Crusade comes to mind where Indy is threatening Kazim on the boat in front of the propeller. The huge prop is chewing and snapping the end of the boat and while it's long, it isn't THAT long. They just used a long lens to give the audience the perception that that giant propeller is right on top of them. The foreshortening could very well alter the audiences perception of the length of the boat. The same could have easily been done with the whip if they were filming in a confined space.
I've shot a number of cracking videos which I've posted on my YouTube channel and often depending on the angle my 8 foots can appear to be 10 foots. It's not uncommon that when filming certain props show as being larger or smaller than they appear on screen. I was floored by how tiny the Chinese theater actually was when I saw it in person in Los Angeles. On TV and in the movies it looks massive but in person it's kind of dinky in size. Same goes for props. Often I am shocked to see how large certain pieces are because the camera needs to pick up the details so they tend to build these things big to make them more visible. You have to also consider that when Raiders was shot the cameras of the time didn't have the same macro focus that most modern cameras have which means it would be harder for them to focus on smaller details the way our modern cameras can.
I think the 8 foot was the predominant whip being cracked in the wide shots (with some exceptions like wraps for that happened moments before a swinging shot - or the aforementioned deleted scene.) I think the reasoning was mostly considerations for space. When you film in a narrow street the physical limitations are going to force you to use a shorter whip.
Notice the shot where Indy does a wrist wrap on the Thuggee guard in Temple of Doom to take the sword from his hand. You can see Harrison's face pretty clearly which to my mind would indicate that he's not standing that far from him, but when they cut to a wider angle when Indy runs after the guard screaming and he runs away as the army of Thungee chase after him, you can see that he is actually carrying his 10 foot because it drags behind him pretty far as he runs out of the frame.
