I would do the whole thing. The writing part is really short. My dd took it a little over 6.5. I would tell the tester to leave this part to the very end. I think you would gather information from that. I don't think it would be an issue for YSP if his scores in written are low, but high enough in the other area or 2.
My dd is not very fast with numbers, and I am glad at the end we did WJ (that has the timed component) instead of WIAT (has not). I think it was good to know that while I classed her as slower, we discovered she is still ahead of grade leve, just not as much.
