I designed my frame so that when I welded on the box section for the X axis rails I could get them as level as possible before welding them and I welded them by tacking and checking the level and then adding more tacks, once I was happy that the rails had not moved I welded over the tacks. One of the box sections does have a bit of twist in it but the epoxy will take care of that. I think if you're selective about the box section you use and are using supported round rail then you can get away without using epoxy. Jonathan's build and Kingcreaky's builds are testament to this.