I recently started assembling my own arrows from raw shafts, but some of the heads don't spin quite right.
The shaft is straight, as far as I can tell, and I put the insert in myself. Aluminum Easton XX75 w/Satelite brand inserts.

Have you ever seen a deformed insert, or does my problem have to be bent ferrule or bad cut on the shaft's end?

I would say it could be many or all the things listed,but sometimes these things happen.You should still get enough straight ones to use for hunting.Save the not so perfect ones for practice

With aluminums you can heat the insert and rotate it until it spins true.That is if you used ferraltite or epoxy for the inserts.Just put in a field point or broadhead and heat the point until the insert is loose.If the cut is not square you should be able to see a gap around the sides of the insert.If this is the case just remove the insert and re cut so it's square.There is no reason you should'nt be able to get it to spin true unless the arrow is bent.Now if you are checking by spinning the arrow on a point,like a top,make sure the point is good and not dinged up.If the point is not straight the arrow will wobble.



CB

try heating the insert and then rotating it a few turns to straighten it up better w/shaft alignment.
Insatiable

Also just by changing the broadhead/arrow combo you can get better alignment. I've taken one broadhead that wobbled like crazy on one arrow spin perfect on another. I just keep swapping until they match up. If I can't get all that I need that way, then I'll re heat the insert.