To key or not to key, that is the question.

I'm getting a new Lang Quick-Point plate to put on my trunnion, this time with a hole in the middle so I can hold long stock through the trunnion. I'm trying to decide whether to key it to the platter of the trunnion or leave it free to be indicated in. Any opinions?

What about a couple of dowel pins instead of a keyway? (If your sub plate allows it.)

Subplate / platter has six radial T-slots, so keys or dowel pins would work about the same. I think I'm leaning toward using six bolts with t-slot nuts rather than three keys and three bolts. My previous one was working okay with just two bolts and no keys. Shouldn't take too long to indicate it in, and I'm not confident the t-slots in the platter are positioned within tenths.

If it was me, I would center plug and rotation key it, but have a little clearance on the center plug, and a bit of slop on the rotation key. Indicate it and mechanically center it, then work offset or grid shift the rotation. You won't be far off enough that it will take any time at all. The most important thing you can do is make sure your rotary is square in the machine and that the face plate is true before you start. If those aren't good its a crap-shoot. People take their 4ths on and off the machines all the time and then chase their tail each time the set the thing up, its because they never square them up to the axis of rotation. They just square up to the not so true anymore face-plate, or rely on the keys to be good to the table after they drop the thing or drag it across the bench. If you want to hold good numbers with a rotary you had better have all your ducks in a row.