If you realistically may not use your miles (after all, about 3/4 of people do not use their airline miles at all) then go ahead and book the domestic ticket using miles. Deferring the use of miles for an international biz/first ticket only makes sense if you will actually book that trip.

I know I will, so that's what I do with my miles, but many a year a go I lost more than 100,000 miles because I was one of those idiots who let miles expire. . . and I now wish I had used them for any purpose, including domestic tickets.