Hi Dale,
I'm no expert, but I usually straighten as best as I can before I seal them. You can still do it afterwards, but you have to be mindful that you don't ruin the finish.
There are a couple of ways that i will straighten. Primarily I will use a large eye hook that is srewed into a 5 inch stick about an inch in diameter.
I will either sight down the shaft or spin it on a spin tester to find the high points. I then will take either the eye hook or just the side of the wooden handle and rub the shaft repeatedly untill it straighten out. I use medium pressure and let the heat from the friction do most of the work.
There are a few different products out there being sold as shaft straighteners but I have not tried any of them. Maybe someone else will come along and be able to help out more.