I have built three Ultimate's one for myself two for other people. The first one was built years ago when the kit was released. they build straight as SPrink has commented.
Silversufers suggestion is also a good one.
There is one area on the Ultimate that I found wanting in strength thats the area where the lower wing fits into the fuse, Depending on what size powerplant you use I have seen the fuse liteply crack in this area on a hard landing. Before I completed mine I used some aircraft grade 1/32 ply strips and reinforced the inside of the liteply wood in that area to take more stress. this is not a big fix but it sure let me breath better on less then perfect touch downs on a rough surface.