It's a line that isn't really there at certain levels (particularly for a vessel as big as a raft) and at other levels it's wide open.
I prefer the right line as a paddle raft line, since you do have the wall. There is a wave in there at some flows (I forget which) that can try to kick you back into the hole in the center, so only having one oar blows in that you can't generate propulsion should you need. It's been a while since I was there, though I believe that anything between 3500 cfs and 6000 cfs and the right line is good to go. Once you're on the line (⅓ to ½ way down the rapid), go ahead and start mixing a cocktail.
Also, the hole in skull is very runnable around those flows as well, though it gets super shallow going over the rock that forms the hole.
One of the beauties of West Water is that it's fun as low as it goes and up to high levels. At 2800-3200 it's got one personality. 3500 another. 4000-5000 another. etc. Different lines/rapids peak at different levels. I find that between 3500 and 4800 the run is usually the most tame, though around 18000 or lots of it washes out (and comes back at higher flows).
The "standard" raft skull line from roughly 2800 cfs up to 20000 cfs is right to left, as others have mentioned. There is also of 'left of center' line at medium flows (no starting out on the right), before the lateral from the left is formed up.
When I was a newbie I watched "Grandma," run the left line with 1 oar under his left knee, and the other oar he only used for 2 leisurely strokes, all while stuffing a rolly cigarette between the tube and the frame. He kept the cigarette dry, pulling it out after the second oar stroke, and lit it up. I say this because it's not all stress. Not all the lines, depending on flow, are wild and crazy.
If the flows are in the mid 4000's and lower, razor rock comes out left of center, and it's a squeeze to go by it.
Attached is a pic scouting skull at 3300 cfs. Helps to see some of the rocks/features.